I feel your pain brotha...at my apartment i get a couple jackasses that do that. So here is what i do since if i get them towed I have to pay for it.
I write down the licence plate number, then leave them a note stating that i have reported their licence plate to managment and not to park in my spot again, i leave my cell phone number and tell them that when they want to leave they can call me. So i park DIRECTLY behind them(sideways if i have to). The only way out is to break my windows and realese the parking brake (which wouldn't be smart cuz i would know who did it), or simply call me so i can meet them face to face and tell them not to do it again.
thats my method, my managment company is usually pretty good about getting repeat offenders towed :D