You disabled registration in the wpmu admin area: Site Admin > Options > Allow New Registrations with that option set to what?
2) Enabled. Blogs and user accounts can be created.
3) Only user account can be created.
4) Only logged in users can create new blogs.
Now you have 4 new users on your site? If you choose option 2 or option 3 then people will still be able to register for accounts on your site.
Are you sure they are users or just people creating comments on your blog posts?