I've had problems with an eBay stalker too, though nowhere near as bad as some!
He bought some stamps off me, then the day they arrived immediately emailed me saying that the stamps were covered in bleach! He also said that he'd checked them all (10 kg or around 50,000 stamps) in an hour or so and that all the postage (unused) stamps had been removed. How he achieved this he didn't say, though he did claim to be bedridden, which made it even harder to believe.
It was clear to me that this person had severe mental problems, so I immediately refunded his money to nip it in the bud.
This however wasn't enough for him. He then left NEGATIVE feedback, even though he'd received 10kg of excellent quality kiloware for NOTHING.
Now every six months or so I get an offensive email from him via eBay's system, which I immediately report.
I suspect he is a wannabe stamp dealer who really doesn't like the prices I get or my crusade against poor quality kiloware and dodgy dealers. Perhaps he feels it's aimed at him.
I never reply to this person, but always refer his posts to eBay. It looks like they warn him off as he goes quiet for a while.
My advice is try to nip these characters in the bud, never engage with them, report them to eBay and don't leave sensitive information on your site.