Let me just say good luck with this. No they should not be parking there but it is very difficult to stop them. We had a row of new houses go up on the opposite side of our alley and the construction workers parked all over our properties, all over the alley, constantly blocking us in/out and left garbage and construction materials and waste everywhere including cinder blocks and screws and nails in the alley drive. It was a nightmare. We put up signs and they just took them out and parked right over them.
Calling the Police did not work for us. We had a medical appointment we were leaving One Hour prior to the appointment, it takes about 30 minutes to get there. We had a car blocking our driveway, we called the Police. They showed up about 15 minutes later. The officer ran the license plate. Went to the owners address down the block from us. The officer came back 20 minutes later and said the owner will move the vehicle. About 20 minutes later a female showed up and said 'sorry'. It was now one hour since we called the Police. Missed the appointment, next one in three weeks. No Ticket Issued, Police said we could NOT have the car towed. We could not drive around the car blocking our driveway as our fence prevented that.
If you call the police and they get ticketed because they are parking illegally, they will stop that behavior.
Signs do not work... But I have some 200 to 400 pound rocks that you can borrow.