Why not gradually reduce the curation reward for posts that have already been curated much? I know there already is a large penalty in voting late, but the OP asserts that it is still profitable to blindly vote for trending posts. If that is true, simply add a factor to the curation reward formula that further reduces or eliminates rewards in proportion to the number of votes the post already has. Whether this be better done by count or by steem amount should be carefully thought through. It might be better to use count, to introduce a little bit of a "one steemer, one vote" element into an otherwise pretty "the rich get richer" themed scheme.