While experimenting with the plugins on one of my sites (trying to see what I didn't need to be running anymore), I disabled Akismet for a while. While it was disabled I noticed that I had nearly 1,000 spam comments sitting in my Comments admin area. So I deleted the Spam. Unfortunately, it looks like Akismet can no longer clean up the commentmeta database, since the original spam messages are already gone. Is there a way for me to clean up the database manually?