This is one of the neat things about a DP/FLEX. After you flex runs for the DP and you have dropped down to 9 on your lineup, you can continue to play the flex on defense. Once the FLEX is due up, you bring back your DP and bump your lineup back to 10 players. If she gets on, put in a speedy FLEX to run again. You then have burned up your DP for the game, but it's a good strategy when you have a DP that can hit, but can't run and your FLEX is fast, but can't hit.
Just to clarify, you have not burned up the DP position, you have simply burned the reentry of the player that was in that position. Once you have established the DP/Flex for the game, it can continue to be used throughout. As long as you have elibible subs you can flop back and forth between 9 and 10 positions in the lineup as many times as you want.