Dean Lewington was the latest to be shown a red card, lashing out with an arm as Town's Simon Ferry attempted to block a quickly-taken free-kick in the 40th minute.
Ritchie made Karl Robinson's team pay with the only goal of the game, an intended centre from the right which eluded everyone on both sides including goalkeeper David Martin. James Collins claimed to have got a final, thin touch but the goal was awarded to Ritchie.
Martin kept the score to reasonable proportions with early saves from Aden Flint, Andy Williams and Raffa De Vita.
Flint missed a clear chance in the second half and Williams ran from the halfway line but shot against the crossbar with Martin beaten.
Dons' best chance came in the last minute when a shot from distance by Darren Potter was fumbled and then held at the second attempt by goalkeeper Wes Foderingham.