By default, the Gmail user interface (including Google Apps Business) contains rotating advertisement called “Web Clips”. Web Clips pulls the information from RSS feed and many end-users may consider it as advertisement.
This is how Web Clips looks like:
While it's possible to disable Web Clips, it being done on per user basis, meaning you (or the end-user himself) need to login as a user and disable Web Clips via Settings/Web Clips page. Off course, you can send email to all your users with instructions how to disable it, but I don't like it too much.
The other solution can be to utilize Email Settings API. It will require some basic programming skills, but at least it will automate the task and eliminate the need to login as a end-user.
However, there is a much simpler way to do the job. You will need to contact Google Support and request disabling Web Clips for your entire domain. You only have to do that before you actually creates any users, as it won't affect existing users in your Google Apps domain.
So, while it's probably won't help you with your existing domains, you can include this step in your domain provisioning flow (as I did to mine) and get rid of Web Clips once and for all.