From beta beat thread:
Jq_sadid:
there no way to undo the curse?
No. They are out for the duration of that Ninja Event. Still can use in every other application of the game. Just no more in the Ninja Event.
Jq_sadid:
Will the cursed hero leave the match right away?
Answered earlier:
Guvnor:
Ckoppe:once they receive second curse… is it an immediate removal from battle, or do they get to finish the level?
Removed as an option AFTER the battle finishes… So you will not be able to use them in future Ninja Tower event teams.