bbp_remove_group_id_from_forum( $forum_id, type $group_id )

function bbp_remove_group_id_from_forum( $forum_id = 0, $group_id = 0 ) {

	// Validate forum_id
	$forum_id = bbp_get_forum_id( $forum_id );

	// Use current group if none is set
	if ( empty( $group_id ) )
		$group_id = bp_get_current_group_id();

	// Get current group IDs
	$group_ids = bbp_get_forum_group_ids( $forum_id );

	// Maybe update the groups forums
	if ( in_array( $group_id, $group_ids ) ) {
		$group_ids = array_diff( array_values( $group_ids ), (array) $group_id );
		return bbp_update_forum_group_ids( $forum_id, $group_ids );
	}
}