If some insults you in a no-joking way, he disrespects you. Would you let someone disrespect you? Of course not. You'd kick their ass. So what do you say? You tell the insulter that if he insults you again you will fuck him up. If he continues to insult you, you kick his ass.
Unless, of course, you are a total pussy and/or your opponent is stronger than you. In that case, you stay quiet and make a plan to fuck him up so bad that he will never ever try to insult you again.
That works for me, anyway.
(Also a protip: never be offended by an insult. It is only a word. It is not the insult that harms you, it is the disrespect that it symbolises.)