Recently, I was told by someone in my playgroup that you cannot take control of a player's commander at any point. If an effect would cause you to control another player's commander, either the effect fizzles or the commander goes to the command zone, I forget which he said.

The thing is, I searched mtgcommander.net, and another website that pointed to mtgcommander.net as the source of the official rules of EDH, and I can't find that particular rule anywhere.

Can someone please clarify? At any time are you allowed to control another player's commander? If so, then the next logical question is if I deal 21 combat damage to a single player with someone else's commander, does that player then lose the game?

It would seem that this question is answered indirectly here and here, but I wanted a more definitive answer to the question.