This is the vBCredits equivalent:
Awards Negation Multiplier:
Whenever an applicable negation award is reversed, this value will be multiplied against the original award and given to the user. For example, if you award 10 credits for a new thread, and want to deduct 15 for having it deleted, specify -1.5. To disable negated awards, use 0.
It's already set to -1.5. Either that's the default and it doesn't work, or you've set it.