This happened to me a while back - the buyer was sus from day one, I blocked them, they got upset and started using other accounts to try and purchase (ones they owned, friends accounts, new accounts etc).
It was very stressful, but I won out in the end. (The transactions I couldn't do unpaid item cases for to prevent the negs, I was able to get eBay to remove them, I also temporarily withdrew all the items they were targeting from sale).
First, go to Account > Site preferences > buyer requirements and activate the block on buyers who have not been phone verified (new accounts tend not to be verified, so that should stop them from creating more accounts to get around it if they try).
Second, as mentioned, contact eBay by phone (or get them to call you back - usually the best option) and explain the situation to them, tell them that since the buyer was blocked from purchasing, you think this is a malicious purchase and intended to cause your account harm by false claims and / or leaving negative feedback. This gets those concerns recorded on file, and eBay can advise you of your options - whatever they tell you on the phone, especially if it's something favourable to you, ask them to put it in writing and email it to you so that you can refer it to them later if needed.
Be persistent if you need to, (with eBay, I mean) if they made the first account unregistered, they are not allowed to open new accounts anyway, so I think eBay should back you up on this one.
