I don't believe there's a way to do what you're asking for just one post/thread. But after you've read the ones you want to read, you can click "Mark All Messages as Read" (or something close to that) on the right, just above the list of threads. The one you're wanting to ignore should eventually die down. Unless it's a journal, then you may just have to delete your reply within that thread so it doesn't show up anymore.