You said it yourself, the buyer used the return reason "Doesn't match description".

They are obviously abusing the process, but there is nothing that you can do about this, other than to report the buyer.

You must accept the return and once received, refund in full.

Then I would suggest that you block said buyer.

This kind of thing happens all of the time.  Buyers do it so that they don't need to pay the return postage.