Help- One of my players is abusing cantrips

Im playing with my party right now and one of my players keeps casting Guidance for every speel and ability check he casts. I think this should not be possible. What are the rules for this? I think it should not be possible to spam cast this spell

Cantrips are explicitly allowed to be spammed. However Guidance has a limited range, so make sure he is always within range to cast it. It also requires concentration to remember that any other spell he's concentrating on stops when he casts Guidance. Guidance also requires a full action to use so he cannot use it on the same turn as casting another spell, or on the turn he makes the skill check. It would take 6 seconds to cast Guidance so in a high pressure situation don't just allow him to cast it instantly. Casting Guidance is also automatically breaks stealth and is very noticable, so NPCs should react to him casting spells (that they might not understand).

But overall, just talk to him. If you don't like Guidance spam then simply ban Guidance and tell him to pick a new cantrip.

Guidance spamming is fairly common, but there’s some things to keep in mind.

First (5.5) it only applies to skill checks, so they can’t add it to a spell. (Or to hit rolls or saving throws, etc.)

Second, it takes an action, so giving themselves guidance on one turn means they’ve used their action and can’t actually make the check until their next turn. So in combat or other tense situations, it’s going to be risky.

Third, they have to decide before which skill they’re using it for. No general purpose guidance. If they choose stealth, but then have to make a perception check, it doesn’t help.

Fourth, it’s concentration. If they’re concentrating on a different spell, guidance ends that. If they do try to guidance themself and wait until their next turn, they make be damaged, or otherwise lose concentration and not get the benefits. On the up side, if they have to make multiple checks with the same skill, 1 guidance will help them with all those checks for the duration (1 minute), at least, that’s how I read it.

Fifth, it has a range of touch, so no helping a different party member from across the room.

Note, in 5.0, point 3 would not apply. You just get guidance, and use it on the next ability check, whatever it is. And you can choose to use it or not after you roll the check. But you can only use it one time.

Cantrips are meant to be used ad lib. It is how many classes contribute when they would otherwise be standing around doing nothing. That said, I don't think they could be using it on any spell they cast. That IS the spell they are casting. In 5.5e, Guidance specifically states that you choose the Skill upon casting and the target gets 1d4 to any Check using that Skill for the duration. This gives the target an average of a +2 to their Check. This is not a big deal. 

If you really don't like it used every time, create situations where there are options for the players to weigh the benefits against risk and let them choose. For example, maybe they are in a place where magic is forbidden and the enforcers of the law recognize common indications of spellcasting. 

Or you can have dungeons with rooms with some indication that magic might trigger a trap because some puzzle was specifically design to frustrate casters. 

Another thing to remember is that they're obviously casting a spell. There are many situations where that's going to attract unwanted attention.