I work for a private library (part of a large university's medical school) and we have the same problem. I don't work in the main building so it's not an issue for me, personally, but I know the administrators have had to balance trying to be humane with keeping the library usable for students and professors. That includes not having bathrooms--which are small, as this is an older building--used as bathing facilities, not having computer workstations tied up by non-students, maintaining a quiet, orderly, environment, not having people panhandling or urinating the shrubs outside, etc. There is a church nearby that runs a soup kitchen, which is great except that it draws even larger numbers of homeless into our facility. My bosses don't like to insist that people leave but we are not a shelter and do not have the facilities to act as one.