This is a wonderful proposal, but I have a simpler idea:
If a listed followee finds a “Spam” proposal, then she can immediately fold this proposal so that her followers will not automatically see it. If a follower wants to see what she folds, then he can just click some button to see the folded; if he think she always folds proposals maliciously, then he can unfollow her or even submit a proposal to unlist her.
I think this idea can cause minimal distortion of the voting market. It is so simple that someone must have already recommended it elsewhere.