First off, who cares about the "rule" that says it can't be done. It's there, stop meta-game thinking, and think in game how to beat it. It can be done under the rules. Munchie? Yes. Unbeatable? No. How? Check the following ideas.
Shield spells counter magic missiles.
Silence spells can shut up the wand user.
Fully concealed, covered, hidden, or invisible attackers cannot be struck by magic missiles.
Magic Missile has a range, 110' min., hit him at greater range.
A blinded (from spell or darkness) wand user cannot target a magic missile, target is considered in total concealment.
Shield potions for the group.
Beating the DM or quitting the table when things look hard doesn't seem to be much of an answer either. Not all of us are surrounded by so many gamers that we can just pick up and find another game to play.