On a mechanically selected transponder (must be flying a DC3 or something) select something innocuous like 2000 0r 7000 or 0000 so when it's switched on next time (or as you prepare the next leg) it doesn't pop up with someone else's squawk. A bit of a worry that something this obvious has to be in sop though, it's just basic common sense and airmanship.
Oh bugger! I've said it again.
I'll get my coat...