...by one of these transients? In a similar situation that I am familiar with, a transient individual rented a unit, then offered the passcode to various others who used it as a base for all sorts of activities- everything from a changing room to ???? In order to fix your problem, I would suggest searching for the source of a possible "community" space, and promptly evict that tenant (following your local statutes, of course). Place signage which cites local loitering ordinance, and ask for police co-operation in the form of routine evening patrols. If your facility is near a local homeless shelter, pay a visit to the administrator and ask his/her cooperation in your communication efforts...

Good Luck with this one...