Gena, if you plan on discontinuing the product entirely then it really doesn’t matter what you do. Just sell out all of your inventory and it will be gone from your perspective. You cannot directly control if there is another seller (hijacker I assume) on the product. They can continue selling the product if they wish (I suspect that they probably won’t if you’re not selling and supporting it anymore).
Regarding ‘bad feedback’, if that other seller gets bad ‘seller feedback’ then that feedback appears on their account and NOT on your account. If it is a bad product review that you are worried about then I say don’t worry. You are never going to sell that product again so reviews for that product no longer have any relevance to your business.
After it is entirely sold out, it doesn’t matter whether you close the listing or delete the SKU (or do nothing)