Q: Our neighbour's Burmese cat is coming into our house through the cat flap and terrorising our cats. How can I stop this?
A: Burmese have got a bit of a reputation for doing this sort of thing!
I would suggest you change your cat flap to an exclusive entry model. Systems such as Sureflap read your cat's microchip number to avoid having to put a big magnet key on his cat collar. This should deter all but the most determined. Unfortunately it's your responsibility to protect your house from invaders, even if your cats can't.