Filtering by /64 is good enough. With a /56 you have 2^8 (256) prefixes, if you spam enough for a /64 to be blocked, you have 255 more tries before all of those are blocked too.

With some heuristics of "hey, we saw two /64's from the same /60" you can catch most ISP's that are offering prefix delegation to their customers, and that's only 16 /60's in that /56 before you are fully blocked...

It's not that much harder or difficult.