Don't allow renewal for same level if user has an active subscription.

my_pmpro_registration_checks.php

<?php

/**
 * Don't allow renewal for same level if user has an active subscription.
 */
 
// Hide the Renew link.
function hide_renewal_links_for_some_levels($r, $level) {
	 
	 // Get user's last order.
	 $order = new MemberOrder();
	 $order->getLastMemberOrder( null, 'success', $level->id );
	 if ( ! empty( $order ) && ! empty( $order->subscription_transaction_id ) ) {
		 $r = false;
	 }
	 
	 return $r;
}
add_action('pmpro_is_level_expiring_soon', 'hide_renewal_links_for_some_levels', 10, 2);

// Prevent registration and display error message when checking out.
function my_pmpro_registration_checks( $continue ) {
	
	global $pmpro_level;
	
	// Get user's last order.
	$order = new MemberOrder();
	$order->getLastMemberOrder( null, 'success', $pmpro_level->id );
	if ( ! empty( $order ) && ! empty( $order->subscription_transaction_id ) ) {
		$continue = false;
		pmpro_setMessage( 'You must wait until your membership expires before renewing.', 'pmpro_error' );
	}
	
	return $continue;
} 
add_filter( 'pmpro_registration_checks', 'my_pmpro_registration_checks' );