MyBB Community Forums

Simple plugin to stop guests view threads on page 2+. It works, but they can still directly access threads on those pages. What can I do?
So far,

$plugins->add_hook("forumdisplay_thread", "pageblock");
function pageblock()
{
	global $mybb;
	if ($mybb->user['usergroup'] == "1" && $mybb->input['page'] != "" && $mybb->input['page'] != "1")	{
		error("Login or register to view more.");
	}
}