Once upon a time there was a plonk on the Usenet. Today there are many other techniques to filter unwanted content.
Please give us an option to ignore user.
Would you mind to elaborate? ignore what?... messages, private topics, mentions?
Posts should not be hide/remove based on this kind of setting, since it will disrupt the conversation going on.
I'm not sure about the benefits, if someone bothers me to the point to ignore him/her I'd rather report him/her to the admins.
You can always leave a message similar to 'moved' or 'closed'. In that case with an option to show hidden content. It works like that in old versions of IP.Board. In IPB4 beta it is possible to choose between things to ignore.
When there is something like 'new content' it can skip content from ignored user. For example a user who use a test forum, is only active in buy/sell category or like to respond in every single topic. Admin could not help when such person do not break forum guidelines.
Definitely messages and/or private topics.