Can you decline a customer’s refund requst

I’ve been doing FBA for years, but started doing FBM recently. I’ve been getting a few messages from buyers asking where their item is as their delivery status is delivered. So far, all the buyers found the items in their mail box etc. I am sure though, I will eventually come across a buyer who even though the item is delivered will state they can’t find it and wants a refund.

Can I deny a refund if the item shows as delivered? Or can I just ignore their messages after replying that the item has been delivered?