From their FAQ:
Simply visit boingo.com/myaccount and choose “Cancel Subscription”. Select your current plan type from the list provided and follow the “Cancel my subscription.” link. Just complete and submit the resulting form with your email, first name, last name, and Boingo username, and we’ll switch your account to “AsYouGo” status in 3 business days. We’ll send you a confirmation number when this is complete. If this option is not available for your plan type, or you wish to completely cancel your account, you may call or email Customer Care (see for all tel/email options: http://www.boingo.com/contact-boingo.php )
One of our visitors said this works fine.