There is the mute feature of course but there definitely should be a block feature as well so that on front ends it wouldn't show the comments at all.
Not on all frontends but frontend only choice.
The only downside I can think of this is if abusers/scammers/whispers would block comments warning others not to get hacked/scammed.
I agree overall that we should have a different function for this, maybe it'd be accepted based on downvotes received by the commenter as that could remove the example above. i.e if I create a comment warning people not to get scammed and my posts are generally not downvoted to 0, then the block feature wouldn't work.
Definitely would require some thinking behind how to best implement it but for your example it definitely makes sense.
I don't think that's necessary. It's not worth burning down the whole house just so a few people don't have to look at the names of a few other people.