Twilight Sparkle: Ohhhh, I get it. It only makes sense that you’d be in charge of the banquet preparations!
Applejack: That’s right! The food situation is handled.
(beat)
Twilight Sparkle: Say, Applejack, would you like to help me look into The Mare in the Moon?
Applejack: Why, of course I would!
DM: Wha-? But…!
Applejack: I heard about Nightmare Moon’s return!
DM: …wouldn’t know about…
Applejack: Sounds like a dangerous character!
DM: …old pony’s tale…
Applejack: I’d be more than happy to offer my services!
DM: …You have to stay and finish preparations.
Applejack: Gaaah, so close. Welp, catch up with you later, then.
Depending on the setting, a character who routinely knows things that they couldn't possibly be aware of might be accused of being a witch, a doppleganger, a spy, or any number of other things. On one occasion where I was DMing, I actually had a character sent to trial and eventually exiled from the continent (i.e. forced to roll up a new character) because his 'foul and unnatural divinations' marked him as being 'in league with Lucifer and his minions.'
Of course, that should be a last resort, for when talking to the player about his/her metagaming tendencies outside of the game (and offering reminders during the game) produces no results. Still, having NPCs react to their freaky knowledge in-character can really encourage role playing!
Yeah, not under the D&D system. While a few other tabletop games (espcially superhero themed ones, oddly) let you take family memebers and contacts as 'risks' so you can get more starting benifits (usually with a cap), in D&D there just isn't a mechanic to make that realistically work.
...Except for, of course, evil mastermind DMs who take advantage of all that extra content you've fed them for their own whims and plothooks. :)
That should be a last resort, for when talking to the player about his/her metagaming tendencies outside of the game (and offering reminders during the game) produces no results. run 3 game online.