CARS ABANDONED?

I have noticed cars that seemed to be abandoned on the premises for weeks now. They are parked in odd places and never moved. What can be done about this? I also notice that someone has taken to the habit of parking at the entrance where it's so obvious there is not to be any parked cars. How can they not notice that if NO other vehicles are parked in an entrance to a community, that is not a designated parking space? So much for guarding the property. This is getting to look like a slum and very inviting to unwanted characters.