Here's a great way to say no:- When asked if you will lend your instrument (especially if its a whistle or harmonica) just ask "You havn't got aids have you? When the reply "No" comes back, you then say " well I have, but you're welcome to play it"
As for melodeons, any one who asks to borrow one should be allowed to keep it, so long as he departs the session immediatley with said instrument.
Seriously, my banjos & Guitars are very dear to me, and are well looked after. I hate anyone else (even my sons) playing them in an aggressive way or leaving sticky fingermarks on them.