Only legacy address space is frequently bought and sold, so it moves between AS#s a lot and is also heavily fragmented.
With v6 this is not the case, a given AS# will typically have a single large allocation and can make it larger if they need to, it won't be sold and moved and an entity can't trade it in to get a different allocation.
You'd typically block an AS - i.e. every IP originating from AS12345. That's just as easy on v6 as v4.