They will probably be seen as viewing something like say going back in their browser or trying to find links and areas from known use, but they will get an error straight away 'you do not have permission to view...' etc.
Are you sure 100% they're in the banned usergroup? If so they won't be able to view a thing and eventually will go idle.
If a banned member logs out they become a visitor and as such can view anything a visitor can view; if they login then you would need to move them into a group that has no permissions to view any content, the caveat there is that they have to be logged in. All they need to do is clear any cookies from their device and they go back to square one - until they login again.