We had a similar problem with our first cat, some neighbours got a kitten, and he started coming in our house, through the small window we left open for our cat, who was a five year old spayed female. Everything was fine at first, and in our ignorance we thought it was quite nice, a friend for Peggy. After a few months she started peeing everywhere, a vet visit ruled out a medical problem, but said it was probably the other cat.

We closed the window to stop his access, but out cat got worse, because suddenly after 5 years she couldn't get out.

So we opened the window again, and had washing up liquid bottles full of water all over, if he so much as put a paw on our garden wall, he got squirted, I felt mean, I liked the other cat, but I liked mine more, after a couple of weeks he stopped coming back, and then Peggy stopped peeing. She lived to be 15, and we never had the problem again. Good luck I hope you sort it.