I used to help run an RPG, and I got so tired of whiny little children. The sad part was that not all of them were children, but they did a wonderful job at acting like it. It wasn't so much a GM scenario issue, but more or less people trying to do things in their RP that blatantly broke the rules or were rather obscene in posts.
I dealt with it by, honestly, putting on a professional, apathetic tone. It sounds bad, but a lot of times I would point out the rules and such and they would get testy with me. I'd give them the "These are the rules, if you don't like it, there's the door." People are just very impatient, and so a lot of times if it was because something on the site wasn't moving as fast as they wanted it to, I would ask them to be more patient and explain why things were moving slower than usual. Either they were understanding or they weren't.
I've had a young cousin, early teens, have trouble in game, as he was having trouble in real life, and working through a lot of issues, and sometimes there were disagreements, fights, his character falling on his sword out of frustration, etc, but we dealt with it and kept the game and real life separate (but of course talked to him out of game), and he eventually gained some maturity and a much more practical playing style.
The most irate playing I've seen from an adult was probably a friend of mine who was a hack and slasher (but had played in other games for over a year and done well) and the scenario was simply going far too slow for him, so he attacked the only GOOD NPC in an evil town - the NPC was a retired Fighter. Once his character died, the player said "Thank GOD! Action! Oh, and I can go get pizza!" The rest of us just sat there at the table, looking at each other in utter bafflement.
I myself got the most irate with the disagreement my ideas for cinematic stunts had, with D20's requirements for Feats and appropriate skills, so once I had been nearly killed for attempting a heroic roll between an Ogre's legs, I began playing my Dwarven Paladin as a juggernaut, bashing down doors and charging into the midst of monsters, sword swinging. Surprisingly, this was the most successful strategy I've ever used in a D20 system (which may or may not say something about what kind of play D20 encourages).