How do I stop listing my ad with a specific website?

f you would like to stop advertising your properties on specific websites:

On the "Manage Properties" page, click on any of your properties.

On the right-hand side of the page, you'll see the list of website you're currently being advertised on. Select the website you'd like to stop listing with and then click on, "Remove all properties from this website".
Click on "Remove all properties".
Note: This will remove all of your current active listings from the selected website.  It is not possible to remove individual ads from a website.