Unfortunately, there is nothing in that feedback which breaks rules so eBay will not remove it.
The only way to remove it is to somehow resolve matters with the buyer. I know they are not being responsive now but the offer of a full refund might appease them, and then you can send them a feedback revision to complete.
If, they do not agree your response qualifies the siituation adequately, and in my opinion would not put anyone of buying from you after reading that.
