There does seem to be a way to ignore users but one has to hover in the post until the flag icon shows up and then the ignore is a dropdown option. What is more concerning though is that it appears many legitimate posts are getting dropped or can't be displayed. I've seen other people report this problem.
Very good question. Appears Yahoo considers junk posts too profitable to advertising to support the Ignore feature anymore. Sad, because the volume of junk posts will drive away serious posters. The original board format was the best. Every "improvement" since has made it worse.