I don't see how it would slow things down all that much. You could even keep it out of the database queries and just do it in the markup. Each post could be given an id or class of the poster's username, and then CSS could be generated such as:
table.posts-or-whatever td#lameuserone { display: none; visibility: hidden; }
table.posts-or-whatever td#lameusertwo { display: none; visibility: hidden; }
That's really the only dynamic part of it. Well, that, and grabbing the ignore list from the db I suppose. Hm, guess it would be more hassle for the db 😛
edit: actually... this would be *almost* doable in CSS by feeding your browser a user-specified stylesheet to use. Hrmmm... might be hard or not possible though.