How do i deal with a pervert?

like on the streets, if they try to hit on you. it happened to me once, by a man that's old enough to be my grandpa! ewww! it happened a like couple weeks ago and i'm still a bit disturbed by that. there's more to this story but i don't wanna go into it but lets just say that i didn't realize it until he started to act creepy. he seemed ok at first but yeah... it was the first time that happened to me so how should i deal, for future reference? should i just tell him to f*ck off or something?