Dealing with illegal parking

Our small church car park is suffering from illegal parking by residents - particularly large vans - that is causing a nightmare for hirers and church users. The council and police can't do anything, and we have looked into parking agencies that fine unauthorised vehicles but are concerned about their enforce-ability. Is there any advice you can give us on how to stop these people parking on our land that become aggressive and violent when we confront them about parking on private property? Andreas Botzios