BANG! Can I use the "Lady Rose of Texas" card to lock out the other player when it is down to two players?
Here is some clarification posted on this card over at Board Game Geek where someone got answers from the publishers.

Looking at the answers that where posted it looks like in your case the card was intended to be used only twice as at that point in the game it is only a two player game.


  Q: Lady Rose of Texas. Can a player use this ability just once per turn, or more often?
  A: Once per turn.
  
  Q: Lady Rose of Texas (and Dorothy Rage). Can this card be used at any point in the turn? For example, can Willy the Kid Bang! the player on his left, switch seats to the right, and then Bang! the player on his right?
  A: Yes, Lady Rose of Texas (and Dorothy Rage) can be used at any point in the turn.
  
  Q: Lady Rose of Texas. Is there a limit to the number of times this can be used in a row? For example, if the game is down to two players, one of them could keep swapping places and the other would never get a turn. Even with the Sheriff and 2 or 3 Outlaws, the Outlaws could keep swapping with the Sheriff and never let him get a turn.
  A: The card was intended to be played a maximum of N times where N is the number of players currently in the game.