A water spray might frighten them off, not sure if it's a long term solution though. Is there anything you can do for your cat to make her less territorial?
There are feral cats all over my garden the place stinks of wee id love ideas too
If you don't have pets yourself you can get the noise making yoke that keeps dogs and cats away, it's soundless to humans.
If your cat is an indoor only cat, you can try sprinkling cayenne pepper around the garden to deter the neighbouring cats.