It may go against the grain, but it is always best to accept return requests. It looks as if you had to refund the buyer anyway and you will have lost your selling fees and got a defect on your account. If a request is opened and you are not prepared to pay for return postage, you can refund and politely tell the buyer to dispose of the item.
The police are unlikely to be interested in your complaint and you won't be able to get the buyer banned. Obviously, on a personal level, you should add them to your blocked bidders list.
What you can do, is contact The National Bullying Helpline. Cyber bullying is part of their remit and they may be able to offer some useful advice.
"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)
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