There's a trick that works for some but not all people. Try talking to the adults with exactly the same phrasing you use for the kids, in regards to responsible borrowing of items. If they bitch, point out that 1) evidently they have not learned the skills so need a reminder, and 2) they can't teach what they don't know, so they need to practice it in order to teach it to the kids.

I've seen my mother shut up a room full of arguing teachers and administrators by turning off the light.