How to remove content from Google?

I am trying to remove an entire folder of thin content from Google to help me recover from a Panda/EAT-related penalty. I want to keep the content on the site for the benefit of users, but not waste crawl budget or have Google think that we have so many pages of thin content.

I added the folder to robots.txt quite a few months ago. While some pages are showing up as "Blocked by robots.txt", the majority of pages now show up in my coverage report as "Indexed, though blocked by robots.txt". About 2 months ago, I submitted a removal request for all URLs that begin with the prefix, but there's been no change. Google Search Console's report updates every few days, but the number of URLs that say, "Indexed, though blocked by robots.txt" is increasing, even months after the removal request for those same pages.