Fending off insults is a bad idea. No matter how logical your replies are, no matter how thoroughly you refuted their arguments, all attending people will just go tl;dl and instead notice how mad u r bro, and being mad is a sign of weakness.
What you could do is try and make them mad instead. Insulting them back won't work, since that kind of reaction is what they wanted to see in the first place, and they have enough self-confidence to be completely unfazed by any kind of insults, which would make that reaction backfire on you.
Instead you could ask for another insult, which will catch them completely off guard, since such people have the tendency to immediately use any insults they imagine right away (also to constantly reuse a single insult), so they'll need a while to think of a new insult, and after a few seconds of thinking you can laugh at them since you just proved they're stupid. Plan B in case they do manage to think of a new one: Just nod thoughtfully, say "Interesting" and ask for yet another one, eventually they'll run out. Plan C in case they don't run out quickly enough to seem stupid: Congratulate them, terminate the conversation and repeat the same procedure a few days later, making sure to point out every time they repeat an insult. Eventually they won't talk to you unless they have a brand new insult, and there are only so many that aren't creative enough to be funny, so you're good to go from then on.