How can you tell which ones are spam accounts? Good start is that they have exactly 100 merit, or exactly 250 merit, and have received nothing since the merit system was introduced. Also that most of their posts are 75 characters or whatever the limit is for a bounty post.
So you could have merit or rank as a changeable thing. Or have moderators with more scope to downgrade obvious spammy accounts, with a warning first then a downgrade (since the reason they are spamming is for bounties). This way you encourage meaningful contributions at all ranks, hopefully reducing spam and with newbie accounts seeing what the new 'norm' is on this forum in terms of quality posting.
Although, this is generally a good indicator it's not 100% accurate. Some members might of contributed with great posts, but since the forum is having a much higher signal to noise ratio they may have stopped. The thing is if they are spam accounts moderators can already do something about it without downgrading their ranks. They can simply delete their posts or ban them if they are particularly bad.
There's a reason why theymos rewarded old members with a base merit, because if they've been here for a few years they have likely contributed something to the forum in terms of posting. Plus, any bad poster before, and after the introduction of merit will not get any more, and won't rank up further. Rank means nothing unless it's for signature/bounty campaigns, and quite frankly if the manager is doing their job correctly then they won't accept someone who is still at their base rewarded merits.
Old accounts with merit aren't an issue in my opinion, and certainly aren't a priority. Also, adding a way to downgrade accounts isn't a very good idea. Even if someone is posting garbage posts without breaking any rules, if they've earned that merit then they've likely made good posts in the past. There's perfect legitimate reasons for the decline of post quality too one example would be lack of time due to work commitments etc.
Word/character count is also irrelevant, and I kind of wish campaigns would just scrap this.
