I know its coming, how to reject without offending?

There is this kid that I am friends with and yesterday I body checked him to see what he would do, and then next cl he's talking to his friends and is like, should i go for her??? and his friends were sarcastically like yeah omg you're so hot and gorgeous you should totally go for her... I admit I've been flirty that day but I don't know how to say no in a way it will offend him or make him feel bad. There is no way in hhheeeellllll I'd go out with him, he is rather annoying and he is ugly, like when I 1st met him at the beginning of the year I thought he was sped by the way he chews his gum. also, yesterday at lunch my friend was like why did you body check her (as in the game hockey) and he was like, I think shes cute. I don't know how to say no in a way that would make him still be my friend....... HELP!