Surely they could refuse to endorse the event, retracting any right to the "Hugo Awards" brand. The event would have to use another name, which would highlight the schism. Obviously a dangerous game, as the committee would attract more scrutiny on its composition and role as ultimate owner of the brand, but probably worth considering as a nuclear option in case, say, "fans" vote for Saudi Arabia next year (yes, "fans" - I'm sure any semi-competent state actor can easily stuff ballots in polls like these...).
That's the equivalent of the "Mike Pence can make Donald Trump president" theory. They administer the trademark according to the rules laid out in the WSFS constitution; they don't decide who is allowed to use the trademark.
The Chengdu bid won the vote legitimately. They would have been massively overstepping their authority to try to prevent it. It's unlikely they could have even won a court case if they tried to prevent it.
