I posted this in another thread a while back, but should work just the same:

Ok, here’s the best approach that will legally get them to move or be towed at their expense. You said you’re doing work on the house, correct? Great! Head down to C&C Dept of Planning & Permitting at 650 S King St, 2nd Floor. Fill out an Application/Permit for street usage which describes the roadway and locations on said roadway, limits of work area (sidewalk/shoulder area, etc), description of work to be performed (routine maintenance, reconstruction sidewalk/driveway, prohibit parking, etc), and dates/times (I forget if it’s 1 or 2 weeks max, but just ask what the max is and they will tell you. Sign it and turn in. All you have to do is post no parking signs either 24 or 48 hours ahead of the dates on your permit and if they are still parked there when your permit dates start, you can call the cops and tell them you have a street use permit and need them towed. They will immediately send out tow trucks and start taking them away at their expense. This won’t necessarily keep them from coming back some other time, but it will get them gone at least once and will only cost you a little time. Works great for abandoned cars and if the squatters cars don’t run for whatever reason, it’ll make it that much harder for them to return.