Can I block a seller from contacting me?

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I purchased an item from a seller that was not delivered as described. I rated him poorly on this matter. He then told me he would send free of charge another item if I changed the feedback rating. I fell for it. He now says he is honoring his side of the agreement but I am required to pay for all of the shipping charges. This is not what I believe to be considered free. I want him blocked from sending messages requesting even more money from me. I am unable to locate a section where I am able to block contact. Please help!