Unless I'm wrong, you are attempting to describe a "Sybil attack", where a rouge entity attempts to force a fault using their majority control of a true P2P network.
While I haven't used ZeroNet myself, from what I understand is that if the network implemented some form of conflict resolution, this wouldn't be possible. As it would be impossible to update a majority share of nodes at the same time, the conflict resolution system would take care of this. One such method is called "Byzantine fault tolerance".
Even thinking about this logically works as well. The hash is already out there for the website, but it "committed to". The website belongs to an entity, and ultimately only that entity can update it (propose a new/unseen version). It would appear that there is a versioning system in ZeroNet, such that another entity could roll the the site back, but not forwards.
Another area to look at is how BitCoins work. Someone can not simply give themselves Bitcoins. These changes have to be verified, and the potential offender can't pick who makes the verification. If someone did own a large majority (+80%), I think people would be skeptical of Bitcoins due to this reason.