That's the opposite of how an ignore feature works.
If you allow members to block content then take the following scenario:
A blocks B
B blocks C
C blocks A & B
D blocks A, & E
E blocks no-one and has no idea that the other members have blocked anyone
A thread with all 5 posters in would make very little sense as significant portions of the conversation will be omitted from each member, except E.
Try reading this thread, removing mine, Kier's and xenDach's posts (including quoted content) for example, on the assumption that we have all blocked you.
Um, blocking or ignoring posts is on a personal level, meaning, if the user doesn't want to see a certain other users's posts, then they block/ ignore that single user and he or she, who is ignoring, wouldn't see the other users's posts. I'd venture to say you understand this Brogan. However, a situation like you have written wouldn't really happen.
It would be more like, say there are users A, B, C, D, E and AH. Things are running nicely, but then AH starts to actually be one. He gets more and more on everyone's nerves, so much so, that A, C, D and E block or ignore AH. His posts are then non-existant only for A, C, D and E. It wouldn't matter what AH says or will say, because he wasn't being productive for the community and its discussions anyway.
It sort of goes along the lines of disciplining, "If you ignore him, he'll stop sooner or later, because he isn't getting the attention he is expecting." It's not the best way to instill discipline or order in a community. But since the users aren't completely responsible for disciplining or teaching other user's to be "good", or at least you can't expect them to, having an ignore function is the next best alternative for the users to help eleviate themselves of having to be annoyed by AH.
Some simple statistics on who is being blocked or ignored within community would also be a good tool for an admin, so he could see who is being troublesome and keep a better eye on them. This is where dieketzer and I disagree. Someone has to get into the fight and straighten things out. When not the admin of a forum, who then? He must be authorative.
Although I too believe, as I think Kier and Mike do, a community should concentrate on the good and positive things evolving within it, there are always "trouble makers" out there and keeping control of the situations they may cause, which disrupt the community, must also be part of a community software. It would be great, if there were no AH's in life. But there are. Having an ignore function is strictly to keep those AH's out there somewhat at bay and it simplifies the admin's life. You could even have an automation/ rule in the system. If 5 users put you on their ignore list, then you will be banned for 5 days. Or something to that effect.
On that note, you guys know "The Annoying Orange"?

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xenDach