When a player has their character do something completely outside of the perceived norm for that character, is it okay to ask them for a justification?
If they can not give satisfactory justification, would it be appropriate to have them take another course of action?
In my group, one of the biggest issues is meta-gaming. Characters doing things that they would have no cause to do, simply because their player has privileged information. It's gotten so bad, my only recourse when I'm DMing has been to ask players to justify their actions. Some of my players are against this; others find it annoying, but understand as they have to do the same.
Even more so, though, we have an issue with people acting... well, random. Quite often, they will pick the most direct route to solve their problems, while completely ignoring anything near standard cultural norms, or even basic common sense. They will do things that, in any form of society, will get them into no end of trouble. Often times, their characters act more like a collection of stock cartoon-gags than actual people. Our group doesn't have a regular DM because of this very issue. No one is willing to try and put up with dealing with the rest of the group as characters.
I tried providing in-game consequences for their actions. They were arrested, and then immediately assumed it was paramount to a tpk. When I, or anyone, tries giving actions consequences, it only ever frustrates people, as one of two things will happen; either they continue acting random and without fail get their characters killed, or they throw on the breaks so hard to do a 180 with their characters' personality that you can almost hear it.
The group averages from 19-24. It's never been larger than six people, including DM. None of us can find a new group; we can never be sure what day we can meet, we're the only players in a ten mile radius, we've all invested time and money into the current game and don't have enough of either to find a new one, and there aren't enough players to give up even one player, as every time we've added a player, they've left within the month because of scheduling issues.
So, we considered just making it a requirement that any given action taken by a character is subject to DM scrutiny, and will be ignored and re-done if found unsatisfactory as something said character might do. Is there any issue with that?