Throw their stuff out, have their cars towed. Legally they are blocking a fire escape which is a huge no-no, and they are not allowed to block your access to your garage due to city, and state ordinances. If you don't want to get your hands dirty, find out who your fire marshal is, and complain to them. They will issue a warning, and give them fines/summonses if they do not adhere to it. It will stop pretty quick after that.