We've been testing our plug in which will have the feature to remove all posts and ban all users that we have in our database. Its still in beta, we've run it on a a couple forums and it banned over 50,000 spam users and removed over 110,000 posts on one of the forums.
Here's the plug in, but we haven't added that feature yet.. so its not going to help you today, but I'll keep you posted.
https://vborg.vbsupport.ru/showthread.php?t=254793