At a session, I'll lend my guitar to anybody who asks as long as he isn't visibly drunk, and I'm there watching. I will take it away immediately if the player - drunk or sober - uses it as a percussion instrument.
This happened last summer at a session in Hoban's pub in Westport, Co. Mayo. A young fella asked if he could borrow my guitar, I said 'Sure,' and handed it over. Well, he started battering the lower bout in between chording (which he appeared to do quite well) and he had a handful of rings, which made it sound like a snare drum.
I instantly put my hand around the neck in mid-strum, and said 'It's a guitar, not a drum.' and took it off him.
He was miffed, but the session players applauded me, so he left in a huff.

Seamus