i have to admit it but (we have cats and we have had other cats come into the house before) the only way IS to scare them, throw jugs of water on them, shout, scream, throw stones (small ones) at them. they stop coming eventually. but be careful when she gets to around a year if you haven't had her done....there will be a herd of tomcats in your garden and house! no point talking to the owner. owners have no idea where their cat goes and what will the owner do? have a word with the cat and tell them they're grounded? some cats are wilder than others and need to be out. just protect your territory!