I am going to get my stickers printed onto HIGH STICK paper, so that they are all but impossible to remove, and my aim is to get them on all the donation boxes, whichwould effectively put the boxes out of action. :-)
Placing stickers on the back of the stall doors in the mens and womens restrooms would be easy enough to do and would give lots of people a chance to read them in private before they were covered up. You could go back every few hours and replace them on some other part of the door or stall wall if they were taken down. Of course now that you've mentioned this here, they'll be a directive to be on the lookout for such activity.