Validation on username?
I don't know if there is anything natively in Craft to do this, but you could achieve it quite easily with a single file plugin:

public function init()
{
    craft()-&gt;on('users.onBeforeSaveUser', function(Event $event) {
        // Only do anything if it is a front end submission
        if(craft()-&gt;request-&gt;isSiteRequest())
        {
            $username = craft()-&gt;request-&gt;getPost('username', null);
            if($this-&gt;isEmail($username))
            {
                $event-&gt;params['user']-&gt;addErrors(array('username' =&gt; Craft::t('You cannot use an email')));
                $event-&gt;performAction = false;
            }
        }
    });
}