Just REFUND them. They can't refuse a refund. Send it back to their SL account and block them, or if they paid by Paypal, click the refund button and send it back to them, then block them.

Then you're done with them. If they post an AB about you, so what? Just show your proof that you refunded them and they were trying to take major advantage of your kindness, and it'll turn into a beware on them instead.