marking territory

[nailak]

A neighbourhood cat keeps coming in to our house at night and missing everywhere and fighting with my young male and spayed female cat. We leave the windows open so our cats have free access to outside and we have a 4 mnth old kitten which uses our garden to pies in.
How do we stop our mummy cats bf from marking his territory in our house?

You need a cat flap that works off their microchips so only yours can come in.

I read last week that they can be fitted in upvc doors if a local company can cut the hole for you.

They are microchipped aren't they?.

i am renting so dont think landlord would appreciate cutting holes in the doors