I installed a cat flap on our door for our cat Pablo, so he can leave the house to socialize with the other outdoor cats in the neighbourhood. We didn't have any problem until one of the other cats (Karel) figured out how to use our cat door. Karel started visiting us quite frequently, mostly for stealing Pablo's food, or sleeping in our bed while we're away from home, but it also happened that he peed on our couch. Pablo is quite frustrated every time Karel enters his territory, but doesn't care enough to scare him away. I tried to scare Karel away by making loud noises, by chasing him, squirting water on him, but nothing worked so far, he still keeps coming back.
I was looking for methods to scare a cat away, but all of the solutions I could find like the infra sound emitting motion detectors and the pheromone sprays or powders would also scare our own cat away.
We talked to Karel's owner, but she is not willing to lock her cat up.