When en passant is the only legal move Consider the following position:
8/7p/5K1k/8/7p/7P/5PP1/8 w - - 0 1

1. g4 hxg3

In the above postion, White plays g2-g4. The only legal move is hxg3, en passant. I heard someone say that since the en passant capture is optional, Black can refuse to play it and have the game declared a draw by stalemate. I think this is nonsense and believe that as long as there are legal moves available there's no stalemate.
Who is right?
You are right, this is nonsense. From the FIDE laws of chess, article 5:


  The game is drawn when the player to move has no legal move and his king is not in check. The game is said to end in ‘stalemate’.


If capturing en passant is possible, it is a legal move so if it is the only option, Black is required to play the move. (Or not to move at all and lose through the clock.)