When you first click on the "remove" button to remove a user, it will display the user's information so that you can verify you are removing the correct user. At the bottom of this page will be a button to Remove User, click on the button once you have verified that it is the user you wish to remove.
Note that everything about the user will be removed: the user's live and expired listings, orders & invoices, communications (both from and to the user), etc.
<tip c n>Note: If there is a lot of data to remove for the user, when you first try to remove the user it may fail because of a "timeout" or perhaps because it ran out of memory. When this happens, it may produce a "blank page", or it may display some sort of "php fatal error". If that happens, refresh the page, and click "ok" or "resend" when it asks if you want to send the post data again. For some users that have a lot of stuff to remove, you may need to refresh the page more than once to get it all.</tip>