Once voted bad enough times it is removed by the network,
Taking over the majority of the MNs would obviously cost money - but we have countermeasures in place to prevent such occurrence, if it is actually deemed malicious (the core team can mark the masternodes collateral as invalid using a spork)

It is not ideal to interfere in such way with the network and fair-and-square kind of situation, but if the whole reason someone setup those nodes was to destroy the network I think it would only be fair to prevent it.