Painfully
First Post
I see it as more of a DM responsibility to set the pace when it comes to metagaming. If the DM won't buy into it, that metagaming is a bad thing, then things aren't likely to improve. Changing things mid-campaign can be tougher than at the beginning. Once you've set the pace for things, it can be hard to get out of that groove. People have probably become quite comfortable with the play style.
Remember that you're asking people to change their habits, and although metagaming isn't quite like smoking, it is still a habit, and it will take time for people to adjust and become comfortable without metagaming. And some people simply won't want to change. Also, people respond better to immediate rewards, than to punishment. Think about that when choosing to give XP awards vs penalties.
If nothing else, call them on it. Say, "your character doesn't know that." Repeat it often, and, if necessary, direct the PC as an NPC until you work past the metagaming situation.
Remember that you're asking people to change their habits, and although metagaming isn't quite like smoking, it is still a habit, and it will take time for people to adjust and become comfortable without metagaming. And some people simply won't want to change. Also, people respond better to immediate rewards, than to punishment. Think about that when choosing to give XP awards vs penalties.
If nothing else, call them on it. Say, "your character doesn't know that." Repeat it often, and, if necessary, direct the PC as an NPC until you work past the metagaming situation.