I don't know how to dump the entire database of users without deleting every account individually. To do that, click on "User List" in the admin panel. For me it's the bottom option in the menu on the lefthand side as seen here.
My default is 20 users per screen, but that can be increased (change the box with the "20" in it to whatever you want). I don't know how high it can go, though.
You should be able to check the box next to each user you want to delete. At the bottom of the screen you can choose "delete" from the pulldown menu.
I'm not sure how to get rid of large groups of people in one fell swoop, though. This way is certainly easier than disallowing names, but it's still tedious.