Just keep "last post" as default order, but ignore all posts made by Newbie and Jr. Member (if necessary).
So thread only bumped if Member or higher rank made a post.
By doing this we won't just be solving a problem which has been needed to be solved for a long time but we will also be catching members that merit abuse just so they can bump topics again. We will likely be able to identify the people behind merit selling and the restrictions implemented aren't that big of a deal. We don't owe anything to newbies and they shouldn't expect to have the same permissions as the rest of the forum for the sake of spam issues sometimes restrictions have to be put in place. I know the community is generally against the idea of implementing restrictions to deal with our issues but that is exactly what this would do. It would solve the problem completely and if it didn't and I'm wrong it would at least be having a great impact on those who can't string sentences together and can't earn enough merit so they go and buy it. The way I see it if you haven't contributed to the forum then you shouldn't have the same privileges as others who have.