There haven't been any more occurrences. Just the two item numbers I supplied previously. And you are right, they weren't blocked before the purchases but they were prevented from buying, twice for one of them, by the Buyer Requirements set on my account, i.e. not having 2 or more unpaid strikes and they appear on my Buyer Requirements Activity Log as refused/prevented. This was on the same day that they later managed to buy. Being a blocked buyer is not the same as being prevented by Buyer Requirements and if the latter had worked as it should there would have been no need for me to block them later.