If you use the greasemonkey script you have an option to disable sites (HN itself is disabled by default). Won't work for chrome, though.
Pages that are dead/not interesting will stop appearing after a month, but interesting pages will linger on. It was important for me to keep the element of serendipity, so I can still find conversations that were once interesting.
Perhaps you shouldn't display anything older than 3 months, as after 3 months one can re-submit the page.