Spraying it with water seems like a good idea. I've had this problem a few times with my cat. At this one house I lived in, the neighbor's cat would get in our house through a hole in the floor in the bathroom (it was a very old adobe house. We did eventually fix the hole) and beat up my cat in his own home. Not cool! So one day, the neighbor's cat got in the house while I was not home. However, my pitt bull just happened to be inside and he liked our cat. I sure you can guess what happened. We didn't have any problems after that. LOL.
In this other neighborhood we lived in, our cat along with every other cat on our block was being bullied by a cat that looked exactly like ours. The neighbors were all mad at us because they thought our cat was beating up their cats. We tried spraying it with water, but that didn't work too well. So all the cats in the neighborhood just had to deal with it. Fortunately, the bully cat just dissappeared one day. I think one of the neighbors probably shot it.
So I guess I don't have a good answer for you. Sorry. Good luck.