Firstly, are the cats causing a problem owned or unwonted animals. If you believe these are unwonted animals you should contact your local council or animal welfare group for assistance in removing these animals to an animal shelter.
 

If you are unsure of who owns the cat a friendly open letter to your neighbors describing the cat and asking them to contact you, popped in their letterboxes may well elicit a contact from the owners.
 

If you know the owners a friendly chat to let them know the cats are encroaching on your property is the way to start. Many cat owners don't realize that their cat is wandering and appreciate the feedback.
 

Discuss with your neighbors the possibility of them keeping their cats indoors. Humane Society has produced a fact sheet on how to train an outdoor cat to stay indoors more, and how to keep indoor cats happy.
 

Cat repellant sprays and gels are available from many plant nurseries.
 

Spraying cats with water from a hose or water pistol once they are inside your backyard will discourage them from returning
 

Place wire at an offset from the top of your fence and cover this gap with soft material to prevent cats climbing over your fence
 

Grind black pepper and chili together and spread the mixture around your garden
 

Placing slow soak hoses along the boundary of your property can discourage cats from entering your property
 

Cats do not like the smell of citrus - scatter some orange and/or lemon peels around your garden (citronella and pine cones are also considered to be good cat repellants)
 

Cats apparently avoid geraniums, petunias, marigolds and fuchsias
 

Plant spiny plants or place corrugated irons around the base of trees to stop cats from climbing them
 

Make sure feeders and bird baths are placed out of a cat's reach
 

Spread thorny branches around your garden