Well, you can go to your ad and modify the title/listing as “Sold,” which will prevent others from emailing inquiries in the meantime. That’s a simple matter of editing what you’ve typed. I’d leave it up as “Sold” until you know it’s gone for sure. Can’t help you with the cancellation of the ad. Sorry.
Click on the link that says 'edit' right under the title of your ad. At the top of the editing page there is a link that says something like "This ad is enabled. To mark it as sold or remove it from the site click here". Then follow the prompts and it will only take a minute to remove the ad, and you'll be asked to pay the 2% fee for sales made on BHO. Zach