As John mentioned below, the listing agent or broker can update the status of their listing within the My Listings section of their trulia account to indicate that a property is sold or no longer on the market. Otherwise, a listing that Trulia receives from a feed will generally remain live on Trulia until the feed indicates that the property is sold or once the listing information is removed from the feed entirely.
A listing that is submitted to Trulia manually must be refreshed within My Listings every 30 days to ensure that the listing information stays up-to-date. the listing agent or broker can remove the home from Trulia. If that's not possible however, you can report the error to Trulia from the property page. While on that page, click on the 'More' tab you find under the listing photos. Then select 'Report an error'. Use the form to indicate that the property is sold and submit the message.
Alternatively, if the two methods above don't work, you can contact Trulia Customer Service team directly for assistance - either by phone at 888-466-3501 Monday - Friday 7am to 4pm PST, or by email via the link below.