The definition of mark room details the amount of room that one boat must give another under Rule 18, but it does not place an obligation to tack on either boat.
Care to try again?
If you're entitled to mark-room over an outside/leeward boat, you can't sail away from the mark just for strategic reasons.
I'll go you one further and say that even if you
AREN'T entitled to mark-room from (and in fact, even if you owe mark-room to) an outside/leeward boat, you
CAN drive her beyond the mark. She can take you head-to-wind, but you are not obligated to tack, nor to give her room to tack.
