How can i stop an unwanted cat?

i am a cat lover and have 4 of my own.recently a cat has started to visit and slowly edged itself into the house he doesn't look very healthy hes got very dry skin and scratches all the time and I don't want this passed on to my cats.I have tried locking the cat flap at night but he just sits by it all night,if he was healthy I wouldn't mind but with 4 of my own i can't afford the vet bill.how can I make him go home (where ever that is), my cats are out door cats so i need to have the cat flap open.