Simple problem, not so simple answer.  We manage a 5plex on a circle with 3 other 4plexes.  All different owners, different dumpster services.  Our dumpster is the closest to the main street and people from all the other buildings pile trash around our dumpster when theirs is full.  My first thought was to get a locking dumpster, but seeing how they throw trash bags at  the dumpster, I don't know if having a lock will make that big of a difference.  I've thought about putting signs on it, but I'm doubtful on that one as well.

Any ideas? 

Obviously, I could put a camera up, but if it's a real camera, and if I catch people dumping, following up on it will be an added job.