To remove a sidebar ad:

1. Go to the 'publish' screen in the Monetization module
2. Select your Wordpress account
3. Retrieve a list of sidebars and select the sidebar the ad has been published to
4. Retrieve the list of sidebar widgets.
5. Select the ad you would like to remove/replace
6. Click 'publish' to replace or the [x] to remove.

If you still cannot remove the sidebar ad, log into your Wordpress wp-admin section in your browser and then go to appearance->widgets and remove the text/html widget containing the ad.