I live in a flat (more like a 2bed terraced house on top of shops) and due to the main road and they are PFBs my cats are only allowed out when dp or I are around. They are very happy housecats with a house full of toys, many different levels to sleep and play on, scratching posts etc. I would prefer not to have a discussion about house vs outdoor cats 
I have lived here 5 years and my furbabies are 4 1/2. There have been other cats around and mine have tolerated them with no issues. About a month ago a fat white cat started hanging around up on the sort of patio area kind of thing outside. Black and white girlie is not too fussed about the great outdoors; she is very shy and retiring generally. Fluffy ginger friendly boy (in process of becoming PAT cat - he's that way inclined 
) has taken issue with the fat white lump and makes the most horrendous yowling noise whenever they meet. He's doesn't seem capable of fending the outsider off, although they have had at least one scrap.
I'm hesitant to get too involved because it's a cat thing to sort out, but the white one was right outside the open kitchen window (outdoor access) this morning, obviously trying to get in. How do I discourage this cat without harming my own? They do need to feel safe and secure in their own home, and I'm worried the noise will disturb my neighbours - two families with toddlers/babies.
Thank you 