In order to cancel your listing on Trulia you must remove it from the site that is sending it to us. Most likely your listings was posted on Trulia through Zillow Rental Manager, which is a partner site that syndicates listings to Trulia and other websites.
You can remove your listing from Trulia by removing it from Zillow Rental Manager: http://www.zillow.com/rental-manager. You can deactivate your listing by clicking on Expire on the Listings page or by using the button at the top right of the listing's review pate.
Your listing will remove from Trulia within 24 hours once you expire it from Zillow.