You could try using a spray bottle. Every time you see the neighbours cat coming towards your entrance, spray it with water. Other than that the only thing I can think of is getting an infrared cat flap that will only let your cats in. Staywell 500 Infra-Red Series

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Owned by Ratty girls Hera, Athena, Boomer and Kitty the Cat
RIP Apollo and Starbuck, together again at the Rainbow Bridge. Till we meet again.