2
a) Only set the right blocks visible on non-forum pages (you can choose, in which modules to show a block, when you edit the block)
b) Add this to the forum code (goes on ALL pages, you don't want right blocks on)
$xoopsTpl->assign('xoops_showrblock', 0); //this goes AFTER the inclusion of XOOPS_ROOT_PATH."/header.php";