What, of course there is. There's a red button in the lower-right hand corner you can hit to turn it off in most cabs; granted, there are some where you cannot turn the screen 'off' but you can turn off the display of advertising on it (and it just then says "Touch here to resume").
That "off button" doesn't turn it off, it makes it goes into a silent slideshow mode showing you more ads with an equally bright screen. Point is you can't win against the ad machine.
