Trespassing cat

[designforlife]

I share a house with two other guys, one of whom is the owner. The owner has a fluffy nightmare of a cat, which uses a catflap in the kitchen.

Unfortunately a neighbourhood gypsy/feral/stray big old tomcat has started to come into the house through the catflap, presumably to eat "our" cats food, and possibly molest it during the day...who knows.

Said ginger tom is full of fleas, and is in the kitchen most times when i come home from work...I can hear it bolt through the catflap as i come through the lounge...by the time i'm in the kitchen i can see it slinking up the garden, pausing occasionally to gloat.

Any ideas how we can stop this cat ingress? Our incumbent cat has been utterly useless at defending her home turf, beyond some half hearted hissing.

Blocking up the flap is a no go, "our" cat spends as much time indoors as out, and does her business outside.