Is it a numbered space, and if so, are you sure that you own it ? - in my apartment complex there are fewer car spaces than apartments, and the number don't match. So someone in Apartment X might assume that he owns car space X, when he might actually own car space Z, or no car space at all.
Does your management agency/company not employ the services of a private clamping company? This solves the problem overnight, because if you leave a note indicating that you'll get the offending vehicle clamped this ensures that it's never parked there again.