I realize that I’m coming in a month late here, but you don’t have to implement Reddit’s horrible system, either.

I’d argue for something like: If you block somebody, then they’d be able to interact normally with your comments that existed up until the time of the block, assuming that they already participated in the thread. Meanwhile, you’d be more restricted, unable to respond to any of their comments at all. So, even if you respond to their comment and then block, because the comment was before the block, they’d be able to respond to it, but their response would be hidden from you, and you’d be unable to respond to it. Anything after the block would act both ways, keeping either user from interacting with the other, except to allow them to block/unblock each other.

There would be some weird ways around it, but I think it would stop the block-griefing behavior that’s rampant on Reddit. If they did use weird techniques to get around it, then admins would just have to ban their entire account.