To expand upon what Susana said, you can't control when ads show up, with the exception of un-checking the box that says "this hub may be considered commercial" and setting your ad-level to none. However if you do this, make sure you read the FAQ to determine if your hub really is non-commercial.
However, you can add your Adsense id, so you earn money off the ads showed on hubpages. If you add your Adsense ID, your google ads will be shown 60% of the time on your hubs. You can't control how they are shown though, because Hubpages does this for you.
To add your Adsense ID, simply goto: https://hubpages.com/my/affiliate