These types of tampering not only violate the terms and conditions of the AdSense program but also often harm the efficacy of the ads, Google said on Friday .

A common tampering practice involves placing AdSense ad code within an iframe, so that it can be displayed on another page, a practice Google forbids.

"Improperly iframed ads are a disservice to our advertisers since the ad itself can be obscured. Not only are they not viewable, these implementations can lead to accidental clicks if these hidden ads are placed in a location that users frequently click," wrote Cecelia Choi from the AdSense Policy team in the post.

A related improper use of AdSense ads is what Google calls "cross-domain iframing abuse," which allows sites to frame ads belonging to another site without the publisher's knowledge.

To make its policy clear in this respect, Google has modified the language in its AdSense policy related to this issue to read: "AdSense code may not be altered, nor may the standard behavior, targeting or delivery of ads be manipulated in any way that is not explicitly permitted by Google. This includes but is not limited to the following: clicking Google ads may not result in a new browser window being launched, nor may Google ads be placed in an iframe."