Mass Member Removal

We are deeply confused (surprise!) about user/member management and I hope someone can help us clear this up. Here is the short version -

The site is using membership. Last year we added 8,894 users to the site for the initial membership test, but didn't roll the site out with membership functions enabled.

In August, we had an issue with users resetting passwords and getting other user information (we have lots of members with the same last names, one of the issues we had to work out before turning membership on) and to stop that, we went in and deleted all membership data manually and all but 44 site user accounts, with levels ranging from super admin to subscriber.

Today I was poking around in preparation for turning membership on and discovered that the 7800~ users are still showing up on the dashboard reports but not in All Members. I need to reset this, quickly - what is the best/fastest/most effective way to delete them? I have fresh membership data to upload, and I want to start with a clean slate to make troubleshooting problems easier for everyone.

Many thanks -