I think I just did it.... We'll see.
I went to services, then TV then "channels" then I could see that HBO was checked, but I couldn't uncheck it. However at the bottom of the page, there was another HBO (along with all the other premium channels) that I could uncheck. I did that and then 'placed order" which was for a negative dollar amount. Got a receeipt saying the channel would be removed on the next billing cycle.