No, not that rule.

My players were following a lead that took them to an underground fight club where criminals and shady alchemists train or blow off steam, and where bookies reign. To enter the club, they were explicitly told to "be a good guest" - which included signing a magical guestbook with a magical quill.

Said quill takes a pinprick's worth of blood, which is infused in the signature and can be used to track anyone who fails to pay their debts. 3 players signed. The bard repeatedly (but politely) refused, then successfully cast Charm Person on the bouncer and shmoozed his way in. When we ended the session, the players were still inside, and there's about half an hour left on Charm Person. So naturally, I want there to be some significant but creative consequences in store after they leave.

This is a universe wherein assassin's guilds are legal and highly regulated (think Discworld). The party actually works for one. Nobody would care enough to put out a contract or risk an illegal murder for a guy who just wormed his way around the guestbook on his first visit. And banning him is counterproductive to the bookies making money. That said, enchanting a bouncer to circumvent protocol is...bad manners. The slap on the wrist ought to sting a little.

Now, I have an NPC named Nana Patient. She's an incredibly old kobold who enforces the undercity's etiquette with her knitting needles and her caramel candies. You don't want her caramel candies. You really, really don't. I'd love to use Nana for this. Maybe she forces the bard to be her personal mannequin while she teaches him manners? Puts him in detention, Harry Potter style? I'd love your ideas.

So, i heard about this little underground fight club where they make you sign a guest book (apparently the bookies hate it when folks welch) and that let's them track folks (at least, that's what they tell ya).

What they never tell anyone, because that would just be bad for business, is that there is a ward around the property to help keep the sorta you don't want coming in and breaking the joint up in a fine and fit state. But, you gotta understand -- it doesn't keep 'em out. It keeps 'em in. If they ain't signed the guest book, they can come check it out anytime they'd like, but, well, they can never leave.

Or at least not until ole Nana Patient is done fleecing them for all they are worth.

I hear tell how that's how they get some of the less fortunate folks for the pits, as well.

Quote from AEDorsay >>

So, i heard about this little underground fight club where they make you sign a guest book (apparently the bookies hate it when folks welch) and that let's them track folks (at least, that's what they tell ya).

What they never tell anyone, because that would just be bad for business, is that there is a ward around the property to help keep the sorta you don't want coming in and breaking the joint up in a fine and fit state. But, you gotta understand -- it doesn't keep 'em out. It keeps 'em in. If they ain't signed the guest book, they can come check it out anytime they'd like, but, well, they can never leave.

Or at least not until ole Nana Patient is done fleecing them for all they are worth.

I hear tell how that's how they get some of the less fortunate folks for the pits, as well.

 I think that is a great suggestion. Only folks with their name (and blood) in the register can leave. It also makes sense from the point of view of the fight club and has nothing to do with the characters. It is a measure to prevent thieves from escaping with the likely large sums of money stored at the fight club. The club would have all the usual defenses to keep thieves out, they would have security, but thieves would have to somehow bypass the book in order to escape and that would be a closely held secret.

Consequences are up to you but being turned into the night's entertainment would seem appropriate and it would be amusing to see when the party tries to leave and three folks walk out while the last one hits a wall :)

I just need to say that Nana Patient is a fantastic name. 

Ok so you’re in a fight club. He’s a bard (NOT A GOOD COMBO). You could make him fight in an arena inside the fight club for entertainment, against some sort of creature, or their best competitor. Win and he leaves. Oh yeah, forgot to mention it’s too the DEATH

OP already said that death was too much for this.  Just make him fight in a jobber match against one of the arena's heavyweights.  Let him get smacked around a few times before the main event.

Quote from David42 >>

Quote from AEDorsay >>

So, i heard about this little underground fight club where they make you sign a guest book (apparently the bookies hate it when folks welch) and that let's them track folks (at least, that's what they tell ya).

What they never tell anyone, because that would just be bad for business, is that there is a ward around the property to help keep the sorta you don't want coming in and breaking the joint up in a fine and fit state. But, you gotta understand -- it doesn't keep 'em out. It keeps 'em in. If they ain't signed the guest book, they can come check it out anytime they'd like, but, well, they can never leave.

Or at least not until ole Nana Patient is done fleecing them for all they are worth.

I hear tell how that's how they get some of the less fortunate folks for the pits, as well.

 I think that is a great suggestion. Only folks with their name (and blood) in the register can leave. It also makes sense from the point of view of the fight club and has nothing to do with the characters. It is a measure to prevent thieves from escaping with the likely large sums of money stored at the fight club. The club would have all the usual defenses to keep thieves out, they would have security, but thieves would have to somehow bypass the book in order to escape and that would be a closely held secret.

Consequences are up to you but being turned into the night's entertainment would seem appropriate and it would be amusing to see when the party tries to leave and three folks walk out while the last one hits a wall :)

 I think is is what I'll do. I forgot that I had originally intended to give them the option of entering a fight, so using it as a consequence is perfect. The bard bouncing off an invisible ward will be enjoyable to narrate. Thanks, AEDorsay and David42! 

Quote from Xalthu >>

I just need to say that Nana Patient is a fantastic name. 

 Thanks! I actually created this NPC for my previous campaign, but the players never explored the area she ruled. Pretty excited for her debut.

As a special surprise, have one of the "early bouts" be some horror show monster type and some proud as a peacock rogue, with the club announcing the match is aa warm up, and the Rogue is someone who snuck in and skipped the signing.

Then watch the Bard's face out of the corner of your eyes while looking at a different player. Or, you know, don't do that. Just let it play.

But do be sure to have a few other folks who thought they were smarter than the system n a lockup -- if nothing else, the roleplay of skeevy types should be a blast.

So, the fight club has four categories: Bones (fist fight), Brains (magic fight), Beasts (proxies, summons, or wild shapes) & Banks (anything goes). The party entered during a Beasts fight, and I ended session with Dex saves from an errant Fireball in a Brains fight. I think I'll have the player roll a d4 to decide his category and fate. 

He's a bard 2/rogue 1 multiclass, so aside from Beasts, he's capable of competing in any category on his own merits. If he does roll Beasts, the party has a familiar and a homunculus that can be his proxy, or I suppose he could convince the warforged monk to take his place. Unfortunately for him, Charm Person was his last leveled spell of the day. So that will make this consequence even more amusing if he has to fight a mage...

As for watching his reaction, alas, I am an online DM. But trust me, after 3.5 years of playing with him, his terrified "oh no" in the voice chat is a very rewarding substitute. ;-)