1. By using the Close This Ad Early link on the ad page (displayed only to the ad owner whenever they are logged in and viewing one of their ads).
2. By navigating to the My Open Ads option on your Manage My Ads/Auctions page (via the Bookmart Optons drop down on any Bookmart page), then clicking the Close button next to your ad's listing.  

Note: If your ad is an auction and it has already received bids, you are obligated to sell the item to the highest bidder at the high bid amount unless you remove all bids prior to closing the auction.  To remove bids on an active auction ad, log in and click on the Bid History button on the ad page.  You will be prompted to explain to the bidder why you are removing the bid.  Sellers who remove bids without a reasonable explanation may be subject to administrative sanction including temporary or permanent account suspension.