If the buyer paid by PayPal, we do get informed by them of any payment refunds/reversals, so in that case we will amend your E-junkie Transaction Log accordingly and automatically expire any links issued for that order. You will see these orders shaded pink in your Transaction Log.
Other payment services we support have no way to notify our system when a payment is refunded, so in that case you will need to expire the buyer's link(s) yourself:
Just click the Transaction ID for that buyer's order in your Transaction Log, which will bring up their thank-you/download page for that entire order -- or if you redirect to a Common Thank-you Page URL on your site, you will need to click the name of each product they'd purchased in your log, which will bring up the page for just that item alone.
Then click each Download button enough times to use up all the Attempts remaining for that buyer's file. You don't need to complete these downloads; just start and then cancel the download enough times to use up all remaining Attempts.