I used to have such problems too. When a user is misusing pending payments to get a download link, the only way to prevent is to go to "Edit Payment Preferences" and check the box for "Wait for pending payments to clear". Also, I deactivated the "let buyers edit quantity" box in the product details. Another additional option in Paypal is to refuse any transactions that are "open" (not instantly cleared like debit cards, e-checks, etc.)