Go to any of the messages they have sent you.
Scroll right to the bottom of their messages and click on 'actions'.
You have two choices from the dropdown menu, 'report the message' and 'block the sender'.
Do both of those things and don't read any more messages that do get through.
If this person has opened a case to say the item you sent was 'not as described', Ebay are likely to find in their favour unless, as @tressygirl suggests, they were too late to open the case.
That, unfortunately is one of the drawbacks of selling on here and there is not anything you can do about it.
You don't have to put up with the nastiness though.
It would help if you gave more information. If they have opened a second case (not as described) and they were in time to do so, you don't stand much chance of winning and would probably be best advised to refund them without paying for a return.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
