How to block a url?

Hey,
lets say that I own http://www.myshop.com
I have a sub-url for selling a product, lets call it http://www.myshop.com/product
It has been online for a long time and its already indexed etc

Lets say now that I stop selling this product and I take it offline.
I noticed that when someone searches in Google for "myshop.com product", the link still comes up (although it returns a 404 error when you click on it)
How can I block entirely Google for returning results for this sub-url?
How can I block http://www.myshop.com/product , de-index it completely?
(using Webmaster tools perhaps?)

You got the idea. Use google webmasters tools. request for a link to be deindexed.

do i file a request or should I add the urls in the robot.txt file??

User-agent: *
Disallow: /product/

add that to your robots.txt