Here are your steps:
Imagine they are toddlers and talk to them like toddlers.
Every time they do it, ensure that you are loud (to attract the attention of EVERYONE, and this is important) and friendly (to show EVERYONE that you are not being a bully yourself).
For example, greet them loudly and happily and say: "Oh wow! I remember making fun of people's names when I was a toddler. I love it. This really takes me back to when I was four. Do it again!"
If they do it again, say, "Hey, that's not bad, but infant humour has a style to it that you still have not mastered. You just look like a cute bunny bouncing around in heat. Try it again but try to be more nonchalant, more suave."
If they do it again, continue to be helpful, and continue to point out their mistakes. "Again, you're getting there. Look, think about the syllables in my name and really give them some emphasis." Actually make fun out of your own name to show how simple it really is (and to show to everyone else how infantile it is) and then continue to gentle help them improve. "Seriously, have another go. I believe in you, mate. I really think you can pull this off if you practise."
And do it loudly. Get an audience so that everyone is watching. If the bullies become violent, you have witnesses.
And if they walk away, follow them and insist on continuing to help. Be REALLY helpful and friendly. "Ah, come on, mate: don't give in so easily. Even my four-year-old nephew had trouble with it, but eventually he got it right."
If the friends join in, say, "Yeah! Well done." And then back to the bully. "See, your friend can do it. Give it another try." Get his friends to help him out.
They'll soon stop.