HOW TO DETER AN INVADING CAT AS IT ENTERS - THE CRAFTY DIY OPTION

The most effective idea to stop an invading cats, if you have patience and guile, comes from one of my readers in Blewbury. He suggests fixing a sensor alarm, which is only turned on while the resident cat is securely kept in a different part of the house out of hearing range. If you don't keep the resident cat well away from the alarmed cat flap, it could trigger the alarm. Then it may never use the flap again and it might refuse even to go into the room with the flap. So the alarm is used as a one-off to deter the invader, and switched off from then on.
First of all study the habits of the intruding cat. Use a battery operated door alarm a magnetic door/window sensor alarm, selling at about £6.99. It is essential that there is an on/off switch with this. Focus sells suitable ones. Fix it alongside the cat flap and only turn it on, when your own cat is safely out of range and when the intruder is likely to visit. As soon as the intruding feline goes past it, the alarm will be activated - loud enough to see him off forever! The key to success is being careful to keep your own cat out of earshot, and having the patience to wait till the intruder intrudes!
Another aid is to invest in a long distance water pistol and ambush the cat. Empty tin cans with pebbles can also be thrown from a window, not aimed directly at the invading animal, but nearby so as to scare him off with the noise. This method may not entirely stop an animal entering but it may insure that at least it doesn't come into the garden while you are there to bodyguard your own cats.
A reader who had a deaf cat, rigged up a portable tape recorder to a motion switch attached to the cat flap, recorded a real din of people shouting etc, and this deterred all invading cats! But this would only work if you have a deaf cat.
Remember - you must NOT terrify your own cat.

IF YOUR CAT BULLIES NEXT DOOR CATS

If you are the owner of an invading cat, then give a water pistol to your neighbours and ask them to use it. Arrange a time share so your neighbours know what time of day your cat will be out, and can keep theirs in. Neutered cats are less likely to be bullies. If your cat has not been neutered, get him fixed quickly. Finally, give information above to your neighbours to help them take precautions such as not leaving food down and keeping the cat flap closed at night.

DETERRING INTRUDERS - MAKING A GARDEN LATRINE FOR YOUR CAT

A series of cat latrines round the edge of the garden may help establish a safe core territory for your cat. It is also the responsible way to stop your cat leaving cat poo in your neighbour's garden - a cause of neighbour rage! Dig a two foot deep hole, fill with one third gravel and then top up that last third with soft playground sand. You can flush away urine but you will have to pick up poo regularly. Put your own cat's poo there first to establish whose territory it is.
Other latrine ideas not as a deterrent but so as to spare your neighbour's seedbeds - a sand heaped under a dry hedge, a generous layer of composted bark under shrubs, a litter tray undercover in a verandah, a litter tray with a cat flap into the garage or shed.