Get some stickers made up (about 12" square is nice), politely reminding them "This is a permit holders only parking area, please do not park here again with out a valid permit" carefully attach these to the offending vehicles windows.
Ok this sounds a little tame but here's the kicker, you have em made with extra sticky glue and out of the thinnest tissue paper you can get. Sit back and watch em spend the next hour peeling off 5mm long shreds of paper which then stick to them!
Its a nightmare trust me! i used to go to a company that does exactly this and got done a few times.*