bbp_remove_forum_from_all_subscriptions( int $forum_id )

function bbp_remove_forum_from_all_subscriptions( $forum_id = 0 ) {

	// Subscriptions are not active
	if ( ! bbp_is_subscriptions_active() ) {
		return;
	}

	$forum_id = bbp_get_forum_id( $forum_id );

	// Bail if no forum
	if ( empty( $forum_id ) ) {
		return;
	}

	// Get users
	$users = (array) bbp_get_forum_subscribers( $forum_id );

	// Users exist
	if ( !empty( $users ) ) {

		// Loop through users
		foreach ( $users as $user ) {

			// Remove each user
			bbp_remove_user_subscription( $user, $forum_id );
		}
	}
}