Why would you want to disable function? If you don't need it then just don't use it.
If you managed to click it accedently few times then add confirmation. To do that open includes/adminfunctions_template.php, find this:
Code:
construct_link_code($vbphrase['revert'], "template.php?" . $vbulletin->session->vars['sessionurl'] . "do=delete&templateid=$template[templateid]&dostyleid=$template[styleid]$LINKEXTRA").
and replace with this:
Code:
construct_link_code($vbphrase['revert'], "template.php?" . $vbulletin->session->vars['sessionurl'] . "do=delete&templateid=$template[templateid]&dostyleid=$template[styleid]$LINKEXTRA\" onclick=\"return confirm('Revert?')").