Help, other cats use my cat flap!
For some cats a cat flap is an open invitation to enter the house and help themselves to the residents' food and even settle themselves on a bed. It could be because they're sociable types, with the attitude that they will be welcomed by humans and have nothing to fear from the other cats.
While harmless, they will no doubt be upsetting the resident cat, leaving him potentially without food and scared of the intruder. The other type of feline intruder may be an aggressive type, determined to steal food and cause a fight - some even spray to assert their domination in the house.
Either way, the resident cat is likely to be disturbed and he or she may suffer with stress-related conditions such as cystitis or begin unwanted behaviours such as spaying around the house to mark their territory rights. Your vet will be able to help you to understand and cope with physical and behavioural problems.
In terms of your garden, look at ways of making it difficult for neighbourhood cats to come into it. If it's out of the question to fully enclose your garden, so creating a safe environment for your cat, look a making your current boundary fences or walls difficult for a cat to scale.
As for the cat flap, you could look at replacing it with a more modern, technical model which will allow only your cat access. There are cat flaps which operate with a small magnet devise which is worn on your cat's collar, while a new concept in cat flaps are those that will operate in response to a pre-existing microchip. They can be programmed to work for multi-cat households and are designed to be strong and secure. Of course, they also have the advantage that you don't have to fit your cat with a collar.
Owners who have upgraded their cat flap report positive changes in their own cat's behaviour and well being as their sense of security is improved.