My PCs will be doing a mission soon, where they need to save a damsel who can - if successful - be a companion in their adventures later. In that role she's a warlock, but for the purposes of the mission it makes more sense that she can't really help herself against her assailants. In a similar, earlier mission the PCs rescued a druid but I had him drugged up past lucidity as the means to keep him helpless. In this case the damsel is being supplanted and kidnapped out of a fancy ball event.

I don't know if there's a spell/spell scroll/other device that can be affixed to her that nullifies her magic skills. I could homebrew something up of course but first thought to see if others had ideas/knowledge that might fit this bill?

Most of the individual effects that would impact spellcasting inflict conditions that prohibit spellcasting, like paralyzed, stunned, or incapacitated, but those might be to similar to your druid scenario.

Feeblemind might be an option, or a geas with the directive to "not cast spells"

Also petrification?  It would be a good way to sneak here out of the ball event...kidnap, petrify, then pose as hired help "just here to move this statue" folks.

Negating magic is a powerful mechanic that isn't likely to be conveniently available at low level. However, you can achieve the effect by blocking the use of spell components:

(S) Somatic - Something like manacles can reasonably prevent a caster from using their hands effectively.
(V) Verbal - A gag or the Silence would both work
(M) Material - Take away whatever the caster uses to focus their magic.

Since these are fundamentally all mundane solutions, I could easily imagine a set of Arcane Manacles that achieve the effect as a single package. The trick is to prevent it from being abused in combat, so say, make it take a minute to properly equip. So, no problem if the target is already bound or unconscious, but impractical in the heat of the moment.

Otherwise, your best bet is to be on standby with Counterspell or Antimagic Field if you can get your hands on it.

If she's a warlock, you could say her patron was mad at her and withheld her abilities from her, for reasons that will play out later in the story, whatever fits the plot. I wouldn't do something similar to a PC, but NPCs don't have to follow the same rules.

Or make the room she's being held it a no magic zone. Like a permanent antimagic field (which is a spell), tied to a ritual or object. But that would be heading into homebrew land. 

You could sort of flip this around a little. She is being kidnapped because the kidnappers believe she is a warlock but she is just a munane mortal, high society gossip can be a horrid thing. During the misadventure she calls out mentally for help and a patron answers and "piff, paff, poof!" the party are breaking down the door to rescue her. The damsel in distress is rescued and goes about helpng the party with her new found powers and firmly believes they were sent by her patron to help her out of a dangerous situation.

You can just come up with something like

Magic Item: Arcane Manacles. Has the effect of normal manacles In addition, it prevents the victim from casting spells and is protected by an Arcane Lock.

Homebrew a collar of "You can't" that can only be unlocked with a special key-gem, that will lose its magic as soon as its opened or your pcs will use it for shenanigans.

My party "arrested" a Paladin NPC.  To prevent him from doing anything while we transported him we bound his arms, ang gagged him until we got him to our first waypoint.  Then we put him in a magical sleep, which would last for a while.  As long as nobody wakes him from sleeping, he's not getting into any mischief.

We did use a silence spell when we arrested him to prevent any magic from getting him out of the arrest.

I have come up with an item for my campaign, points if you can work out where I got the influence from :). 

In my world one particular empire "Etresh" uses sorcerers as living weapons on the battlefield. It controls them with a metal collar which locks in place and nullifies their powers. They are unable to remove the collar without suffering extreme psychic pain. The collar has a matching bracelet that another user can wear, this then allows them to "tap" into the sorcerer forcing them to trigger and cast the magic when the "owner" decides. 

You could come up with a similair thing that is targeted just at a warlock, an item that cuts the warlock from their patron stopping the magic, or seriously limiting it. 

there are also nullify magic spells, it would be easy to have an npc give them an item that replicates that magic, maybe in a 10 foot sphere around the item, it gets attached to her and she cant cast magic. 

Keep it simple, old school Medieval.  Fingers and tongue removed.

Just to keep it interesting...

Have them inject her with a rage potion.  Rage stops casting and concentrating. 

Since she is a caster, princess at a fancy ball, she'd probably has a terrible strength score. And if she's in a corset, ball gown and heels, that like halfway to being restrained already.

Quote from Bodanger >>

Just to keep it interesting...

Have them inject her with a rage potion.  Rage stops casting and concentrating. 

Since she is a caster, princess at a fancy ball, she'd probably has a terrible strength score. And if she's in a corset, ball gown and heels, that like halfway to being restrained already.

 Oh. While a "Rage Potion" might not actually exist, modifying the character in a similar way is a good idea.

Something like Polymorph would absolutely do the trick, and would also make transporting them much easier.