What can I do about homeless trespers?

I live in Connecticut in a four apartment building. We have back porches, with outside access to all four apartments. We have a problem with homeless people coming onto our porch and stealing things. Usually it's just soda cans that they return for the deposit, but they take other things as well. Last winter I put up a No Tresping sign, and that seemed to deter them. Until the sign went mysteriously missing. I later found it buried in the snow and rehung it. It keeps falling down. I think these homeless people are taking the sign down, so they can claim they didn't see it. I am about ready to super glue the sign to our stairs. I am so sick of these people coming onto our porch and taking our things. We also have small children living and visiting here (all under 5 years old), and that makes us very worried. What can we do?