You can block this person's communications - go to the site map at the bottom of the page and under seller stuff there's a block for bidding and communications, put those in place first.

Don't do anything else. If she lodges an "Item not received" case or whatever, phone eBay (not email or live chat), they can see all of your eBay communications and can remove bad feedback if necessary. Under the "resolve a problem/contact eBay" tab there are a series of tabs that eventually lead you to a phone number and give you a once-only code to use to talk to a human being about the issue.

Don't respond to any more of her communications - I have been through this sort of rubbish with difficult buyers, and if your communications have been via eBay's message system, they can see them and usually support you and remove any negative feedback. If they don't, calmly ask to speak to a supervisor.

Good luck,

Marina.