On-line retailers have rediscovered an old tactic called Inertia Selling. It involves billing the customer for repeats of the initial purchase unless the customer specifically cancels them. As the name implies it works by busy customers forgetting to cancel by the due date. Back in the pre-EFT days it eventually faded out when savvy customers just didn’t pay and some consumer protection laws were introduced.
Now the retailing trick is to label every purchase as a “subscription” and include a clause like “all our subscriptions are automatically renewed unless you cancel within 14 days” at the bottom of the home page or worse buried in the T&C which no-one reads. If the customer made the initial purchase with Paypal and has funds in their account or a valid credit card attached, Paypal will continue to pay the merchant and reject any request by the customer for refunds on the basis that the payment was “authourised”.
The only answer I can see is for you the customer to make sure only sufficient funds to cover the purchase you WANT to make are available in whatever account Paypal makes payments from.