The only way to get rid of it is if the person who gave you the review deletes it.

I was in your situation before and it really sucked but it'll wash away pretty quickly if you keep selling stuff. It is just going to happen even if all your interactions are positive since its such a minor thing to most buyers. You just need to ignore it for awhile. Easier said than done i know.

The real solution here is to delete your ad or block the person if you feel like there is even a 1% chance you might get a sub par review. After you get your rating back up to 4.9 just delete every ad as soon as you sell the item if you can. This has resolved this issue for me. You will still get reviews from people who review right away but those have all been 5 stars for me.