Go to "User Administration"...then, from the column on the left, select "Management". In the blank box beside "Look up user" enter the name of the user that you want to delete from your membership list and click on the "Look up user" bar. Once you have the user information showing up on your screen, go all the way to the bottom and click on "Delete this user?" (Note that this cannot be undone so be careful to make certain that this is what you want to do). The unwanted user will then be completely wiped off of your site. Hope this helps :D