When a new user is registered on your Wiki, the user automatically goes into your Wiki's XWikiAllGroup group. It is possible for you to remove a user from this group.
Assuming your Wiki's URL is http://mywiki.xwiki.com/, go to
On this page, you can click the user's URL on the right, then click the "remove this object" link above the field where the user's name appears.
If you want to add back a user, select XWiki.XWikiGroups from the Add Object popup menu on the right, then click the Add Object from this Class button. A new, empty user object is created, and it appears as an empty, numbered link at the bottom of your user list. Select the link, and in the now empty Member field type the precise username, which must start with "XWiki.".