QGA--What do you do if Black tries to keep pawn?

I've played a rapid game 3 days ago where Black tried to hold onto the Pawn with ...c6 and ...b5 and he lost very quickly: http://www.chess.com/livechess/game?id=1231220228

Thanx all 4 the help! Checked out Skynet's game, very interesting. Anyone else have games along those lines?

1.d4 d5 2.c4 dxc4 3.e3 b5 4.a4 and there is no holding on to the pawn for black as far as I know without losing material.

Doesn't 3.Nc3 allow 3..Qd6 where that isn't blacks best move probably but it looks like it allows them to keep their pawn.

It's not a trap though... It is simply a response to the bad move 3..b5 which shouldn't even be played.
4.a4 is the best move as far as I know.
4.b3 looks weak in comparison.
nimzobogo you said in your annotations that 3. e4 can lead to a crazy gambit line... What is that line called? I usually play 3. e4 so I want to know just so I'm not caught off guard

The purpose of 3.e3 is to just take the pawn back, the trap is just icing on the cake. It's the most solid option and teas positional with the 3.Nf3 line.
Trouble with 3. e3 is you've gotta be prepared to play against 3...e5 and get an IQP (although White often gets one anyway in the QGA, but normally gets a bit more compensation when he does)
I know its wrong for Black to hold onto the gambit pawn in the Queen's Gambit Accepted but what is the best way for White to proceed?
For Example:
White to move.