I agree about the magnetic collar thing, but here's a funny aside to your question - I've got 5 cats and about a quarter of a mile away lives a nasty tom cat, who used to get in through my cat flap (even when it was locked!) and terrorise my girls and my young (neutered) male. All the girls are spayed as well, by the way. The bl***y thing used to attack mine when they were outside as well and I had several trips to the vet with them, having been bitten by him. I asked the owner, nicely (at first!!) to get him neutered and she said she would, but months went by and he still came over here & attacked mine and got in the house and sprayed. Anyway, one evening, this thing pursued one of my girls in through the cat flap - he'd only ever been in before when I was out. What he didn't realise was that when I'm at home, the dogs are also in the living room and he flew through the cat flap only to be confronted by a rather large GSD and a crazy collie (and 5 cats who were intent on beating him up!). The cat flap was locked to "in only" and this brute did a wonderful impression of Garfield, trying to get out. He then went & hid under the dining table, surrounded by my "pride" and their canine minders! He didn't even put up a fight when I scruffed him and stuffed him in a cat box & unceremoniously dumped him back home. he was neutered shortly after that & we haven't had so much trouble from him since. Get a dog!!