An old "friend" wont stop pestering me?

dont really know how to word this. but someone i had a one night stand with wont stop asking me to do it again with him even though i have a boyfriend and i have been with him for 2 years. i was single and he is my cousins friend and lives in london and i live in the eastmidlands. he had a business meeting one day my way over 2 years ago and we met up went shopping and in the moment made love in his car. im not going to lie he is gorgeous but it was just a once off a bit of fun. i like being friends with him but every few months he will facebook me asking to do it again and next month he is down my way again. he wont take the hint and is very persuasive and tells me nice things about me to win me over. what girl doesnt like being complimented hey??? i cant tell my boyfriend cus his answer will be to knock him out what can i do??