Interestingly I had this too on Sunday.
Player heckled another player (opposition) after a shoulder to shoulder and called him a fa**ot (which I personally despise!)
I called the player over with his captain and the conversation went as such:
Me: "Thats the last time you use that word on my pitch"
Him: "What? Fa**ot?"
"Yes. And Ill say it again - thats the *last* time you say that word"
Player walks away - and when about 10 foot away says (first aimed at his team, then turned to me);
"Im not allowed to say Fa**ot... But you can eat fa**ots?"
"So you're saying you want to put that player in your mouth?"
Blushes and laughter from his team.
Yes, I just ended up ridiculing him, but if I was in a worse mood - I would have sent him after line 2.