Here's one strategy for handling returns from unhappy customers:
Let them know you don't accept returns. Explain that it must be a user error. Explain that the customer must have lacked care or intelligence or ethics. Explain that you're willing to accept a return, but just this one time. And finally, explain that you're now going to put the person on a list, and you'll never sell to him ever again.
Do all this in one continuous statement, without pausing for a response.
This has happened to me more than once.
What puzzles me is this: if you're going to give the customer a refund, why not make them delighted by the process? Why not create an aura of goodwill? At the very least, both of you will have a better day. Even better, perhaps one day someone will mention your company to this former customer–I wonder what he'll say?
One tip: if you say your meta-goal out loud (or jot it down) before you start an interaction, you're more likely to consistently create the outcome you seek, not the one you hyperventilate yourself into.