"Of course, one solution for the Spam problem would be for it to request a small one-time account setup payment from users, either by credit card or Paypal"

Unfortunately, that could make things worse. The same criminal elements that are behind the spam and the "products" it purports to sell would have no problems using stolen PayPal or card details, particularly for a one-off payment. Then the poor ISP gets not only the same level of abuse complaints but also chargebacks for fraudulent transactions.