I hope this scribbling makes sense.

For keeping the obtained rank you should get a certain amount of merit points. The gradation could look like this:

Junior Member: 2 merit points a month.
Member: 4 merit points a month.
Full Member: 10 merit points a month. And so on.

I might be a bit too ambitious on merit point, numbers look quite high and can be reviewed.
This should eliminate the bountyspam from users who just keep doing that because they can, bounty hunters will just lose the rank and ending up being Junior Members/Newbie either improving their post quality. This system also requires additional actions against merit abusers.

The most real control instrument looks like health system from some RPG.
By not receiving enough merit points the account will lose, figuratively speaking, "health". After losing 5 health point in a row account automatically lose 1 rank (de-ranking).

We can also safeguard the old inactive users, the ones who are inactive won't be affected at all. On the over hand, for the users which are willing to take a rest from the forum and obviously don't want to lose a rank, we can create some sort of "afk" button. By entering it you won't be affected by monthly merit points requirements, on the over hand in this mode possibility of writing the messages is equal to zero.

That's just a draft, don't judge me too hard.