I just say no (unless it's someone I know).

If they cop an attitude, it's their problem - they asked, I answered. I don't want to be disliked or get abused but if they haven't got enough manners or whatever to respect your feelings, who cares what they think?

I've found it's usually a waste of time reasoning with them as someone who would ask and not take no for an answer usually has no idea what a person's instrument means to them. If they get insistent, I just tell them to push off (it helps that I'm 6'3 and 280 lbs).

The worst are the ones (and you know who they are) who ALWAYS turn up at jam sessions without an instrument and want to borrow someone's. I've got no time at all for these losers (I'm venting a bit here but it's one of my pet peeves).

By the way I've been playing Bass and Guitar professionally for over 20 years and about 90% of the damage to my instruments has been caused by "borrowers".