digitalv - 8:17 pm on May 18, 2010 (gmt 0)

Take it out of any category listings so you aren't showing unavailable products to visitors who are browsing, but keep actual product page up with an Out of Stock or discontinued notice, and make sure you have "related products" or "similar products" on the page.

This way if a search engine decides that page is the best match and drops a user on it it will still be there, but it's not in a category where someone can click through to it.

You already captured that visitor - if you don't have EXACTLY what they were looking for, at least try to show them something else that might also be a fit.