Tighten the PMPro Spam Protection so users/IPs with failed payments are locked out more quickly.

tighten_pmpro_spam_protections.php

<?php
	// Add these 2 lines to your wp-config.php
	// This will lock IPs that fail checkout 5 times within 30 minutes.
	// The default is 10 failures every 15 minutes (900 seconds).
	define( 'PMPRO_SPAM_ACTION_NUM_LIMIT', 5 );
	define( 'PMPRO_SPAM_ACTION_TIME_LIMIT', 1800 );  // in seconds

This recipe can be found on this post: https://www.paidmembershipspro.com/how-to-stop-spam/