How do I stop other cats using my cat's catflap?

I have a timid female cat and recently put a cat flap in. Unfortunately other cats are now coming in. I have chased cats out and they haven't come back, but one cat in particular just will not get the hint. I once even cornered him and poured a jug of water on him but he's been back. Are there any tried and tested (humane!) ways of getting rid of the unwanted cats?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).