From your comments I take it that you don't think there is anything else that can be considered (aside from the merit system) that would increase the quality of the discussions? It would be disappointing if the quality issue is just a fact of life which simply has to be accepted.
There probably are ways to go about dealing with the spam, but I don't think this can be done without restrictions which affect legitimate users. At the moment we've only stopped users who post bad quality posts from ranking up not removing it. Generally though I think the forum is in a much better place than it was a year or two ago.
There's a constant juggling act when implementing restrictions and dealing with the spam. If there was a solution which couldn't be abused and wouldn't limit legitimate users that would be great and would probably be implemented in a heart beat.