Advice on how to tell creepy guys to f*ck off?

I'm constantly being approached by men that want to flirt with me and its beyond the point of being obnoxious. I have no problem with being blunt and rude but I'm afraid "fuck off" isn't always the magic words for the more persistent, stubborn men.

I have no problem with someone trying to have a conversation, but usually any seemingly innocent convo turns into flirting. I'm in a committed relationship already, so not at all interested in anyone/anything else.

Ladies... what has worked for you?