eBay does not really care about the QUALITY of the feedback, especially from Buyers.
So you are stuck with it - all you can do now is attempt to have the feedback withdrawn using the mutual agreement process.
Well worth it since eBay is currently enforcing the Seller Non-Performance clause and a negative definately counts towards your assessment.
If you rack up enough negatives, neutrals and open disputes they may ding you and suspend your selling writes for minimum 30days.
Regards,
TIM @ FTL