Place this code snippet in your themes functions.php file to restrict myCRED from giving points for forum actions.

Restrict bbPress Forum Points

/**
 * Restrict bbPress Forum Points
 * If a forum has the custom field "NO_MYCRED_POINTS" set to "yes"
 * no points are awarded for any action taken in the forum.
 * @version 1.0
 */
add_filter( 'mycred_add', 'mycred_disable_bbp_forum_points', 98, 3 );
function mycred_disable_bbp_forum_points( $reply, $request, $mycred ) {
	// If already declined or bbPress is not installed, bail.
	if ( $reply === false || ! function_exists( 'bbp_get_topic_forum_id' ) )
		return $reply;
 
	// Only apply for bbPress
	if ( in_array( $request['ref'], array( 'new_forum', 'deleted_forum', 'new_forum_topic', 'deleted_topic', 'topic_favorited', 'new_forum_reply', 'deleted_reply' ) ) ) {
 
		// Get Forum ID
		if ( in_array( $request['ref'], array( 'new_forum', 'delete_forum' ) ) )
			$forum_id = $request['ref_id'];
		elseif ( in_array( $request['ref'], array( 'new_forum_topic', 'deleted_topic', 'topic_favorited' ) ) )
			$forum_id = bbp_get_topic_forum_id( $request['ref_id'] );
		else
			$forum_id = bbp_get_reply_forum_id( $request['ref_id'] );
 
		// If forum has custom field, decline points.
		if ( get_post_meta( $forum_id, 'NO_MYCRED_POINTS', true ) == 'yes' ) return false;
	}
 
	return $reply;
}