When someone changes their username, the block follows them. However, when they close their account and open a new one, the old ID is not linked, correct.
It's an issue but, if you ever run into a problem and know that the customer was on your BBL, support should be able to resolve this for you.
It is not really the ID that is prevented from buying, it is the buyer him/herself and there are eBay rules governing this.
Bernie