The Help Center FAQ have been updated to include the new block feature.
I was pleased to see it says
Can I block any user?
You can block anyone who has previously sent you a message or placed an order with you.
So, unlike many of us thought, that means you can block users who haven’t ordered from you yet, provided they messaged you and made you want to prevent them from ordering.
I couldn’t confirm it, as the block button only shows up intermittently for me (currently not) and I only looked at actual buyers when it did, not at people who just sent a message. Well, I hope it will be in a “show phase” once I’d actually need it.
Note: This feature is not yet available to all sellers.
(plus, at least for me, it’s available only sometimes) so if you don’t see it, it’s probably, hopefully just a matter of time.