My son did the same except with us it was ____@____.com First, with him, he liked getting a reaction out of us. Of course, the first time they say it we reacted and they're looking for that reaction.
I first told him (and continued to over the years) that when he says words like that, that it makes him really ugly and that is not the beautiful little boy I know. It has worked and worked well over the years. Even now at 12, he rarely to never curses (and I've had older kids tell me he won't even say them when I'm not around too)
After I spoke to him about it, from then on I stopped reacting completely. As if he said nothing at all too. I only reiterated "that word makes your face ugly when you say things like that" when a new one would (and new ones will) popped up. This all helped.
I forgot to add because it has been so long but some words required "the hot sauce treatment". When he wouldn't stop saying a word, I would put a dab of tabasco on his tongue. It won't hurt them like soap does but it works (until they develop a taste for it :)
I'm adding and editing here since I've been accused of being "sadistic" for using a dab of food product to correct a bad behavior. (really? sadistic? give me a break. Tabasco is FOOD. If I made him eat an olive-which he would tell you would be tortuous to him- would I be the Marquis de Sade? BTW, my son LOVES hot sauce now, puts it on a lot of his favorite foods)
First off, we didn't teach him the word he was using, another child at preschool did. He did not learn the word at home. We talked to him about it several times and he would continue to use it at the school just not at home around us but you can only get those phone calls at work so many times before you have to do something.
Again, tabasco sauce is food. It is not harmful, it will not make them sick and you don't pour it down their throats, just a slight dab on the end of the tongue.