<human>: We (=my parents and me) have two cats, brothers and caponized, 10 Years old. We have a non electronic cat flap, so they can go out whenever they want and are used to it since they were little.
The problem is, that since a month or so a straying cat started to come through the cat flip into our home.

Since we thought it was because of the food, we put the food away when we go to sleep, so the cat won't come at night, but unfortunately, now it comes at day.

The cat is shy and vanishes immediatly, if a person appears. Both our cats won't defend their territory (they did so against other cats)

We somehow fear that the straying cat might start to pee into home at some point and might have fleas and such.

Is there an effective way without changing our cats life too much to make this cat stop visting our home?

Thanks in advance!

If I'm not in the right subreddit, it would be nice, if you would tell me, which one I should use. Also I don't know, if this kind of situation was already discussed. Sorry, if so.

Could you distill the essence of the text into 1-3 sentences?
<bot>: a wild cat comes visting our home through out catflip; both our cats wont defend their territory. How to stop it.