You may add recognition ads to pages and then decide to move them to a different page. This is how you can remove an ad which has been placed on a page.
1. Go to the page spread where the ad is placed. Select Content.
2. Select Recognition Ads.
3. Click on the ad on the page. Drag it to the left to drop it back in the view of the ads. Now the ad will not show as Used and can be added to a different page.
Note: Do not delete the page spread to remove an ad. When you delete the page spread, all of the ads will move to Unused and you will have to place them all on pages again.