We are using Google Apps for Education at my school. Here's the ad I was just confronted with when I logged in to my e-mail. This after we have opted out of ads. You are unable (to my knowledge) to opt out of web clips.
It is frustrating that we don't have better control of this.
update: it seems that each user can turn off web clip under mail settings - web clips. I don't imagine that most would ever make it that far.