There should be a link to end the listing (click More Actions or the like to see more links) in your list of active listings on the Seller Hub or My eBay: Selling.
If you can't find it there Go to the Site Map (link at the bottom of this and most ebay.com pages) Sell Section (use the shortcut at the top--it's a long scroll down) and you'll find the link to that (which should give you a list of your active listings to choose from or might require you to copy and paste the item number) and several other useful seller tools.
