Another idea would be to not only require the 25 posts per week, but also require that your 25 posts receive at least a certain number of merits. This should directly stop the shitposting.
I have always suggested this idea. There should be a requirement of getting certain number of posts for all ranks in a year. This makes sure that they keep posting quality content here. Otherwise, a member may continue with shit posts after achieving desired level with few quality posts.