My first idea was an inactivity filter that killed both the vessel and the Aspect, but that doesn't really fix anything.
I like your idea better. Maybe: anyone who has 10 or more 100+ word posts gets a vote taken off them the next exe. Something like that.
I'm still thinking about this. On the one hand, PMs are fun. On the other hand, the nature of this game puts a ton of pressure on just a few people, and PMs add to that. That said, I think the reward is greater than the risk. Like you said, so many possibilities open up for elim gambits. Also, it's likely that PMs won't be there late game, where people have the most info.