Set up a remote access system on client/customer machines for tech support. When a customer doesn't pay, and notices have been sent and not replied to, and they won't answer your calls: this, on all their machines with past due payments.

Then when they call you in a panic, give them the same kindness and respect that they have given to you, down to the number of days since contact was stopped. Gotta twist that knife for maximum effectiveness. Then and only then should you consider answering their cries of agony.

(now I've never had a client payment issue, usually it's quite some time before they need my assistance again so I take payment in full at completion, not tabs/payment plans; but hypothetically...)