Play Strahd as disinterested in actually killing them. He's a god compared to them, they're not threat to him, so he's just showing off and playing with them.
Have him make nonleathal attacks, effectively pulling his punches. Nonlethal spells, too. Give him large untyped penalties to attacks, DCs, and damage (-5, maybe) because he's just playing and not really putting any effort in (untyped so that the players can still attempt to impose circumstance penalties).
Functionally, respec him as a Level 10 creature for the fight. If the players run, let them go. If he knocks everyone out, either have them wake up where they fell 15 minutes later, or throw them in a dungeon cell.