Visiting cat - can anyone advise?

[noarguments]

Since our next door but one neighbour moved in their cat has decided that it can march into our house, bully our cats, eat their food etc. It wanders right round the house, when it gets the chance. Our two seem to be powerless to stand up fro themselves and run away (or hide behind our legs!). Visting cat is brilliantly audacious - it doesn't seem to matter if we're in the kitchen at the time - it will still have a go. We chase it out, have tried spraying it with water etc. We've also managed to lock it in (by locking the catflap one way) to give it a scare, and it was mightily scared, but it seems to have forgotten.
My cats don't wear collars, and I'm trying to avoid getting one of those special catflaps that only let your cat in, rather than all the cats in the neighbourhood.
Does anyone have any suggestions or experience of things that might work?
TIA

[Sparklingbrook]

If you get a catflap activated by your cat's microchip they won't need to wear collars.

[cozietoesie]

Many posters use and swear by the \link{http://www.sureflap.co.uk/\Sureflap}. Just under £70 online.

[noarguments]

Wow, just another example of technology leaving me way behind several years ago. Looks perfect. Will send the bill to the neighbour (not really!)
Thanks. Mumsnet is awesome.

[TalkinPeace]

another vote for microchip catflaps

[Lovethesea]

Ours works great. Only issue with old neighbours cat was that it just tried wandering in the front door after us instead!