Moneypenny44

Hello everyone! I am new to this site but wondered if anyone could help me with something. We have two ginger pussers (neutered girls), aged about 13 (they were rescued as young cats). Last year an aggressive tabby moved in next door. He terrorises them by chasing them. He then is always in our house eating food, spraying and generally being unpleasant. We all feel under siege. We keep the catflap shut as much as possible but our cats are used to roaming freely. My cats don't generally wear collars but when we tried the collar/unlocking catflap idea, the flap wouldn't open. They aren't chipped either but we have been told chip/unlocking catflap does work. Has anyone had any success with this? Any help wouldbe greatly appreciated - my pussers are getting very tetchy and its costing me a fortune in disinfectant!