To disabled the un-register buttons on your site - go to admin > builders > navigation builder and un-check both member and guest check boxes from the unregister link. This will prevent it from being displayed in the top menu and the quick links menu.

You will then need to also get rid of the link from the members menu too.

To do this you will need to edit your database - look in the sys_menu_member table for the entry named Dashboard - Then you will need to edit the PopupMenu field - simply remove the following text, leaving the rest intact

    'unregister' => array(
        'url'     => 'unregister.php',
        'icon'    => 'memeber_menu_sub_unregister.png',
        'caption' => _t( '_Unregister' ),

Then delete all files in cache folder except for the .htaccess file.

Refresh and enjoy.