Change the default PMPro WooCommerce bahavior and make membership products not purchasable if a user already has any other membership level.

my_pmprowoo_is_purchasable.php

<?php
/**
 * Change the default PMPro WooCommerce bahavior
 * and make membership products not purchasable
 * if a user already has any other membership level.
 */
// First disable the core PMPro WC callaback and set up our own.
function my_pmprowoo_init() {
	remove_filter( 'woocommerce_is_purchasable', 'pmprowoo_is_purchasable', 10, 2 );
	add_filter( 'woocommerce_is_purchasable', 'my_pmprowoo_is_purchasable', 5, 2 );
}
add_action( 'init', 'my_pmprowoo_init' );
 
// Our new callback.
function my_pmprowoo_is_purchasable( $is_purchasable, $product ) {
	global $pmprowoo_product_levels;

	// Bail if PMPro or PMPro WC is not active.
	if ( ! function_exists( 'pmpro_hasMembershipLevel' ) || ! function_exists( 'pmprowoo_cart_has_membership' ) ) {
		return $is_purchasable;
	}
	
	// Not purchasable for some other reason.
	if( ! $is_purchasable ) {
		return $is_purchasable;
	}

	// This is the main line that was changed from default PMPro WC
	$has_membership = pmprowoo_cart_has_membership() || pmpro_hasMembershipLevel();
	$product_id     = $product->get_id();
	
	$is_purchasable = ( in_array( $product_id, array_keys( $pmprowoo_product_levels ) ) && true === $has_membership ? false : $is_purchasable );
	
	// Add the warning message for the single product summary page(s)
	if ( false === $is_purchasable ) {
		if ( pmpro_hasMembershipLevel() ) {
			add_action( 'woocommerce_single_product_summary', 'my_pmprowoo_purchase_disabled' );	
		} else {
			add_action( 'woocommerce_single_product_summary', 'pmprowoo_purchase_disabled' );	
		}
	}
	
	return $is_purchasable;
}

// Show already have membership version of the disabled message.
function my_pmprowoo_purchase_disabled() {
	?>
    <div class="woocommerce">
        <div class="woocommerce-info wc-nonpurchasable-message">
			<?php _e( "You already have a membership level.", 'pmpro-woocommerce' ); ?>
        </div>
    </div>
	<?php
}