Filing a false return request

[bavenue]

I have a buyer who has filed several return requests stating the items are defective (without saying how they are defective) for items purchased. The claims are in retaliation for a previous return request claim he filed that I refused. One major problem is that one of the items he filed a claim against has not even been delivered to him yet. Ebay has still allowed this claim to be filed. In the recent past, I have been able to request a call from ebay but now they keep showing a message that they can't call today try again tomorrow. That's fine except the same message was given on Friday. What are my options since I can't contact ebay and I can't refuse the return of an item for a defect when the item has not even been delivered to the buyer (so how can he possibly know if there is a defect or not)?