[calorie]

Untested but try adding the following plugin at the newthread_start hook (how to add a plugin):

// change X to the forum ID of the signature forum
// change 7 to how many days to prevent new threads

if ($foruminfo['forumid'] == X)
{
	$sigforum_check = $db->query_first("SELECT COUNT(1) AS cnt
		FROM " . TABLE_PREFIX . "thread
		WHERE postuserid = " . $vbulletin->userinfo['userid'] . "
		AND forumid = " . $foruminfo['forumid'] . "
		AND dateline > " . TIMENOW . " - (86400 * 7)
	");

	if ($sigforum_check['cnt'])
	{
		print_no_permission();
	}
}