As both a player and GM I play with my group of players (and close friends) on multiple occasions and they will often make fun of either my characters, NPC or a specific situation and won't stop punning it. For example I played a Minotaur Paladin (I loved that character concept a LOT) and players just made fun of it all the time calling me a cow at the table.

They do this also when I'm the GM. Sometimes the name of an NPC will be turned against me (the GM not the NPC itself) and I find it annoying because now I have to watch both my character concepts for their game and also my NPC name because they'll just make fun of it all the time.

So how do you deal with immature players? How do you make them understand that their humor is a form of metagaming (by this I mean...you know that I won't start a PvP combat, but your character sees a Minotaur ready for combat and you insult him all the time).

As another example: one of my player insulted a Drow Matron and it basically brought all the attention to him and he died because the Matron decided it was enough and wanted him dead. The player whined for 2 weeks that I (the DM) killed his character. So when you insult a Chaotic Evil NPC level 12 you expect to live to tell the tale?