Does Curse of Exhaustion and Possibility Storm hard lock your opponent out of casting anything?
It's not a total lock, but it's a pretty strong lock. If they cast anything from their hand, then yes, Possibility Storm will exile it and give them a replacement that they can't cast. Note that Possibility Storm only applies to casting from their hand, though. There's plenty of mechanics that cast a card from other zones (e.g. flashback, suspend, madness) that can get around this combo.