There are several things you can do.
a) if you don't care for land: do a suicide topfeeding wave when their armies are out for more than 2 hours, then withdraw with the honor.
b) if you don't care for honor: stay in war and slowly ruin them, take all their acres, make all your provs unbreakable, then feed on them. In the meantime look for one province who would surrender. Let him grow, while take the land from the others. When he is over 50% of his kingdom's NW, his vote is the only one that matters. He votes himself king, then withdraws.
c) if they have more than 5 in vacation mode, report them, as the maximum provinces in vac mode is 5, and they are exploiting a bug to avoid WD. In the meantime keep ponding the others, plus search for people who would WD. You might get lucky and with many provs in vac mode, you might get a strong enough opposition to outvote their own monarch, then surrender.