Before the forum we have today we were able to ignore posters, we also had other forum features such as downvoting a topic.
The reason downvotes were taken away from us is because downvotes did nothing productive for forum discussion or the posters and readers, in fact, it hampered discussion so greatly that Blizzard was forced to remove it. Ex-CM Ornyx gave reasoning when he stated:
"Yes, we’ve recently pushed an update to the WoW forums that removes the ability to downvote posts made by other players.
We originally added this feature as a means for players to assist with forum moderation by upvoting helpful posts and downvoting inappropriate or toxic posts. In practice, however, we found that it was primarily being used for things like downvoting a post simply because they disagreed with it, which was not the intent, and too often led to different opinions getting unfairly buried. Moving to an upvotes-only environment will remove this unintended abuse, while still allowing players to give recognition to posts that have a positive impact on the World of Warcraft community.
As a reminder, you can still report inappropriate or abusive posts to the moderation team by selecting the “Report Post” option located in the dropdown menu on the right of the offending post."
Fast forward to the new forum we have today and we’ll find many threads that were spam-flagged while said threads clearly broke no forum Code of Conduct rules. Many posters will flag a post that they simply don’t agree with, resulting in that thread being locked down for a time period, which, in turn, can end up burying the post behind dozens and dozens of other threads on the forum - in the end real discussion was impeded.
If we were allowed to simply ignore posters that we wanted to ignore than many posters would ignore posters due to that poster not having the same line of thought as that poster. The end result would cause an actual echo-chamber, an echo-chamber is the most counter-productive weight that a forum could have.
A number of posters simply cannot be trusted with tools like an ignore feature and still be able to have a forum with healthy discussion. If a certain poster bothers you then stop reading their posts and threads, stop giving them attention, and stop giving them your time.
Sources:
https://us.battle.net/forums/en/wow/topic/20757587012?page=1#post-8
https://us.battle.net/forums/en/code-of-conduct/