You left a follow up but you could have sent the buyer a message asking if they would accept a feedback revsion.
Personally, I do not think this was left maliciously, but the buyer was asked to leave feedback and did, not realising his ambiguous post could be detrimental to a seller's account.
A neutral though has no affect on your % mark.
If the buyer still will not complete a feedback revision you can still send to them, contact eBay Customer Services direct them to the message set by the buyer to cancel this sale, and if they can remove the feedback left.