Limit Subscriptions to one active/suspended per user.

one-subscription-per-user.php

<?php
/**
 * Plugin Name: User Limited Subscriptions
 * Description: Limit users to only one active or suspended subscription
 * Author: Matt Allan
 * Author URI: http://www.prospress.com
 * Version: 1.0
 * License: GPL v2
 */

/**
 * Limit customers to one active or suspended subscription. Once they cancell their subscription
 * or the subscriptions expires, they will then be able to purchase another.
 *
 * @param bool $purchasable
 * @param WC_Product_Subscription|WC_Product_Variable_Subscription $product
 * @author matt@prospress.com
 */
function mc_one_subscription_per_customer( $purchasable, $product ) {
	$user = wp_get_current_user();

	if( in_array( get_option( WC_Subscriptions_Admin::$option_prefix . '_subscriber_role' ), $user->roles ) ) {
		$purchasable = false;
	}

	return $purchasable;
}
add_filter( 'woocommerce_subscription_is_purchasable', 'mc_one_subscription_per_customer', 10, 2 );