This is a territory issue. Some cats are not content with their own garden, they like to 'expand' their empire, and ensure that the incumbent cat knows they are there! The best way is to stop them getting together - lace your boundary with pepper, moth balls, or one of the commercially avialbale repellents. Concentrate on the side with the agreesor - if it's your neighbour's cat, place your deterrent on their side (assuming they are equally keen to keep their puss on one piece!), if it's your cat who has the superiority complex, put the nasty stuff around your side. Either way, they won;t be keen to cross over simply to ensure that their territory is defended, they may expand elsewhere though, so keep your eye on future bahviour.