The Reaper lands and character/player A. Player B casts the spell Misfortune on player A (player A must take a 1 for his roll) Player A casts Lightning on Player B thus stunning him. The question is...does player A still have to accept the Misfortune spell? Thus dying as per the Reaper Chart. Timing is everything I guess but player B did cast on Player A before he made his dice roll and before he casted the Lightning spell. I understand Player A casts a stun but Player B had already casted his spell. It doesn't seem like it should negate the Misfortune spell and that player A should have to accept his 1 as his next dice roll.

Player A cannot prevent the other player from doing something that player already did, all he can do is cancel the Misfortune spell with a Counterspell or similar. In this case, he dies:)