The definition of ahosi kamma is defunct kamma which no longer bears fruits. As we know every action has its consequence. From Buddhist point of view, AFAIK, it's impossible to remove any vipaka completely. If an "old-kamma" can become exhausted, then what is the difference with no consequence?
If a kamma is inhibited by a more powerful one, then the consequence is still there, maybe it is minimized or delayed. But if an action has no consequence, I think it's hard to understand it from the law of cause and effect point of view.