I would say, disband the party, but that couldn't be done due to (supposedly) being constantly at war with either Eurasia or Eastasia, and then they would simply absorb what was Oceania. So, what could be done? The whole world was too deeply ingrained in the one party system, so it would have to stay that way, at least until one of the other nations conceded, but then, would the other two?
Also, it's probable that Big Brother doesn't even exist, and is just a symbolic representation of the party.