Consider letting them change the way some spells work — with a couple of caveats.
First off, take a deep breath. You’re doing fine as long as your players are having fun.
This solution is not going to work if you are playing Adventurer’s League or other organized play, since it is not based off of the rules as written (RAW); if that is acceptable to you (and to your group; as always with things that come up in play, the first step is to check in with your group), then consider this: would it harm anything to let it happen?
If there’s no obvious harm to anything, with the possible exception of the plot, and it seems a reasonably appropriate effect that’s roughly in line with the spell’s power and theme (I wouldn’t allow produce flame, a cantrip, to have the same effect as a 3rd level fireball, for instance), I would inform the group that this is something the spell shouldn’t do by the rules, but let them do it anyways. If the only thing it harms is the plot, take a deep breath and realize the plot doesn’t matter if everyone has fun.
This is a way of letting them have some fun with their spells and giving them a cool story to tell —- one of my most memorable RPG stories is “that time we used create water to blind a dragon”. If it harms nothing, why not let them do it?
If you think they’ll abuse the new spell effect, you can say “this is okay, but only this once. Next time it’s not going to work”. If it’s a little out there (as I would say your example is, since they seem to be trying to extrapolate a new effect from the spell), perhaps they have to make an Arcana check or a casting ability check. I have a group that has a standing rule of “if it’s not in the spell description, you have to be clear about what you want to do and why, then it’s an Arcana check against DC 10+the level of the spell”, which has worked well for us since we tend to bend the magic rules a lot.
If it harms something that isn’t the plot, such as a character, a player, or someone’s ego, then tell them it will cause harm and don’t let them do it. However, I would caution you not to use this very often, and to save it only for times when it absolutely, no doubt about it, will hurt a player or a character physically or emotionally. If it isn’t an appropriate effect for the spell (too powerful, too different from the spell’s function, etc) then tell them the spell can’t be changed that much.
It’s okay to say no, but do consider that, in the example you give, they might not have another spell that would work. In your specific example, I would either make them roll an Arcana check (see above) or suggest using a different spell, possibly modified (such as identify, for which I would have them make an Arcana check to identify something that is presumably non magical).
However, I would always check with your group before letting any of this happen. As a group, you have to be alright with spells being modified to do different things, and also with the specific spell in question being used for a specific purpose other than the one in the rules. If somebody objects to any part of it (it may be unlikely in this case but it could happen in others), then you and the group should reevaluate what you want to do about that use of the spell and try to come up with a compromise.