How can I keep a tom cat away from our house?

I have 4 cats, 2 were mine and 2 my partners, we then moved in together so we have 4. We keep them in the kitchen at night and back utility room, but we have a cat flap so they can go in and out for a tiddle. BUT! we keep getting a un neutered tom cat keep coming in and spaying in our kitchen! We tried spraying... show more I have 4 cats, 2 were mine and 2 my partners, we then moved in together so we have 4. We keep them in the kitchen at night and back utility room, but we have a cat flap so they can go in and out for a tiddle. BUT! we keep getting a un neutered tom cat keep coming in and spaying in our kitchen! We tried spraying him with water, but he just don't care! our cats have been done, but he still comes in what can i do to stop him? We don't want to lock the cats in or out overnight. Please someone help, I'm at my wits end with this CAT!!
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