As a temporary deterrent, can you print out some notices to stick under the windscreens of the offending vehicles reminding them that the parking bays are for residents only, and that if they persist a company will be employed to clamp offenders?
If the residents can get together, I guess there is nothing stopping you from parking your vehicles immediately behind/in front of them and blocking them in, which would be very inconvenient when they just need to leave for work, etc. Maybe if this happened a few times they would get the message and stop parking there.