Ways to stop male cats (who are not mine) "scent marking" inside my house - they have access because my cats have access to outdoors with a cat flap

Question

I've been having a problem lately with 1 or 2 specific male cats in the neighbourhood coming in to my house via my cat flap and scent marking/urinating on vertical surfaces to (I presume) mark 'their' territory.

My 3 cats seem to tolerate the main culprits, I wouldn't say they are "best friends" but they do hang out harmoiously out the back!

I don't want the scent marking to happen any more because the smell is really noticeable and disgusting. (the cats don't seem bothered or avoid those areas or anything, though).

Please can you give ideas of how I can stop these guys from doing this?

Edited to add: every couple of days I clean off any existing 'deposits' using disinfectant etc. I bought a UV light to show up where any specific 'deposits' are!

Edit 2: I'm particularly looking for ways that even if a 'stranger' does somehow get in the house.. how to get them to "not" scent mark, rather than how to stop them getting in, if possible, please.

Answer 1

You can get battery-operated, electronic cat-flaps that will only unlock if they detect recognised microchips. This is a convenient method to control access if your cats have been microchipped.

The SureFlap is one such cat flap. We have one, and it works as advertised. You can also get tags with a built-in microchip that can be attached to a collar, but having your pet(s) microchipped is the better solution. The SureFlap is supplied with one such tag, iirc. The SureFlap has a timer to deny entry and/or exit if you want to curfew your cats. It will also allow unknown cats that have managed to gain access to your premises to exit at any time.