What I have done in the past is have the insta-death spells reduce the character immediately to -10 (or whatever threshhold we happen to be using for death). They and their party then have 1 round to stabilize the character or death follows. A healing spell or potion is required to stabilize, not 10% chance of automatic stabilization, no healing skill, etc...
If they stabilize, they will remain unconcious for at least 10 rounds (If the healing spell brings them to 0 or higher).
This ensures that powerful spells are still powerful, yet don't totally wipe out a character with no recourse. It also forces the rest of the party to choose priorities in the midst of combat that may not be beneficial to winning the combat, but that's life.
If they stabilize, they will remain unconcious for at least 10 rounds (If the healing spell brings them to 0 or higher).
This ensures that powerful spells are still powerful, yet don't totally wipe out a character with no recourse. It also forces the rest of the party to choose priorities in the midst of combat that may not be beneficial to winning the combat, but that's life.