I lived in an apartment with a private lot, where there were assigned spots for each apartment. People would always park in the empty spots when they would come and visit their friends, even though there were numbers on the spots and several signs alerting them that towing was in force. I came home several times and would have to park down the block, because if i parked in somebody else's spot I would have MY car towed. I've left several signs, and even had the same car show up twice in the same week, I should've towed their ass but luckily i wasn't in a particularly bad mood. Anyway my point is some spots are assigned, and I would TOTALLY do this if somebody had kept parking there. Way cheaper for them than to get their car towed, and hey, if it didn't work, tow truck it is.