There is a setting which limits how many posts can be deleted at one time with the Delete All Posts button. The default is 15 posts. It’s here in your Dashboard: /admin/site_settings/category/users. Scroll down to delete all posts max and change that. If users have a lot of posts in a topic, it could take a while to get rid of them all if it’s set to a low number.
If a user has uploaded a file and it is deleted, it’s not permanently deleted until after a grace period has expired. (Think Windows Recycle Bin.) The default number is 180 days. This can be adjusted here: admin/site_settings/category/files and scrolling down to purge deleted uploads grace period days.