Users cannot be created by default. To allow the creation, go to "Users -> Users settings" page, check "Enable users creation" and fill in the "Registration mail" witch is used to send the confirmation email.
Note on the wiki deletion
You can delete a wiki with the web interface in: "Wiki -> View all wikis"
If you click on "Delete", a confirmation window will appears. If you click on "Ok", the wiki will be deleted (see the following "Note")
Note: The deletion will just put your wiki in a "deleted state". It means that your wikiboxes contained in the wiki will be available if you are linked it in an other wiki, and all wikipages attached to your wiki will be marked as deleted (cannot access to them).
Note also, that you can "undelete" a wiki.