It took about a day for ServerPronto to reply that my cancellation request has been accepted. It looks like I'll be off the hook next month.
Maybe I got lucky. Maybe all the complaints come from people who didn't send the correct documentation. Maybe ServerPronto has made an effort to improve lately. Whatever it is, I can't complain.
I'm on to larger hard drives, faster processors, and bigger bills. It was great while it lasted, ServerPronto. Thanks.