[MENTION=23935]Nagol[/MENTION]
I can't say I disagree with anything there. Your approach is just different. And I'm sure it works fine for your group.
In the case of the player with wizards A and B, the co-author approach would be:
Wizard A casts fireball into a melee for the first time; the group learns that they don't like this. He tries it again later. One or more other players chime in and say, "dude, please don't play your character this way, it's going to get someone killed; why would he do that? Can he wait cast it later?"
The ball is now the wizard player's court. If he wants to work things out with them, he would A) explain why his character is doing it (maybe his wizard is dumb, maybe scared, maybe he's chaotic, maybe there is no character reason, the player just didn't know he's making other players mad, etc, etc) and b) be willing to talk alternatives
1) "Well my character is terrified, and when he is he throws fireballs; I wanted that to be his thing" The players work out together some ideas on how the nervous wizard can be portrayed but without making other players angry by constantly harming them.
2) "My wizard is barely a wizard because his int is so low; he doesn't realize what he's doing" The players suggest that he's probably smart enough to learn from the angry comments the characters made the first time he did it, and they work together on ideas for a dumb wizard that don't make the party angry
3) "Oh, I didn't even think it hurt your characters that much.... I can stop" The player simply realizes there was a misunderstanding.
All of the above can sometimes be worked out IC, of course. Some people are skilled at that and some aren't.
It works just a well in character. Almost the same words in fact. There isn't much skill required to use personal pronouns rather than "your character".
"Wizard, you caused more damage than the orcs. Why did you do that?."
<insert Wizard's response terrified/ignorant/callous/whatever>
"Wizard, you are still mailing all the melee fighters with your damage spells. You're going to cause a complete failure someday. I am not amused."
<insert Wizard's response terrified/ignorant/callous/whatever>
"No this time we mean it. Find some other way to be effective in combat."
<insert Wizard's response terrified/ignorant/callous/whatever>
"Thank you for all your efforts on the party's behalf, but we don't think the adventuring life is a good fit for you. We've tried, but your fear/stupidity/callous disregard for our safety -- despite our repeated warnings -- cannot be tolerated in the field. Here is your share of the group pot. We hope your next endeavour fares better."
OOC,
Wizard player "Really guys?"
Group "Yep. We kept warning you. You're lucky B didn't kill you in your sleep on his watch after that last stunt. His character is seriously pissed. What are you going to bring in next?"
Wizard player "A different wizard. This one will be more of a team player, I think. We need the arcane power."
Group "Sounds good. Make sure we don't have to abandon this one, will you?"
The time to use OOC conversations is when those conversations can't happen between the characters for personality or other in-game reasons. For example, if an antipathy is starting to grow between two characters and one or more players is uncomfortable with the dynamic. At that point, it isn't an in-game problem it is an out of game problem and needs to be addressed there.
"I noticed our characters are butting heads over things that don't seem worth the trouble. Any idea why? It'd be nice if you can bond as friends, what circumstance would work for that?"