If you refunded then you presumably accept that the item was faulty. The buyer has left factual feedback and has no reason to revise it just because you refunded them.  Many buyers would consider revising if they got a refund but they are under no obligation to do so, the fact you refunded does not change the fact they got a faulty product in the first place.

Your best bet is to leave a short follow up, something like "Sorry, full refund given as soon as you told us" or similar would be best.

Many of us base our decision to buy not on if a seller has negs or not but how they handle them, in other words if they reply with capitals and name calling we avoid!