I propose a feature where a user can specify a list of users whose questions, answers, comments, and profiles should never appear.

If that's too much work to implement, as an alternative, I propose tagging all questions, answers, and comments with a class specifying which user it's from: e.g., posts from me might have class "posted-by-13". I could then write a custom stylesheet to cause those classes to have "display: none" in their style.