I can't remove the channel - unlike other channels, I am not given that option when I press the * key when HBO NOW channel is selected. I suspect the reason for this is that HBO NOW is a subscribed channel so I probably have to cancel the sub, then remove and then reinstall.
If you subscribed through Roku, yes.
I do have another question, though. After you've logged in to HBO NOW, and when you're done watching, how do you exit the channel? Are you hitting the Back button or the Home button?