OK - Finally found the Settings/Preferences. Then, toggled the Startup view app settings from on to off. When we opened the Video app after doing this, the ads were gone - YAY!
But then when we opened it again, they were back - BOO!
It appears that the ads come back after the initial opening of the file, so are not permanently removed. And, if you tap the 'home' button on the upper left of the app it goes to the ads page, not to your personal videos.
We also tried logging out of MS account, and closing the Video app in Task Manager. The ads still appear even when we are offline.
This tablet is being setup for a relative who is always offline and only wants to look at their own videos - not have to wade through store ads.
Anyone have any other thoughts on how to permanently hide/remove the store ads?