If your ROKU was installed by someone else and you don't have access to that ROKU account, you should perform a factory reset. That way, your personal account will be on the ROKU and you can add or remove any channel you want.
To conduct a factory reset, follow these steps:
Current Roku players (Models 24xx, 25xx, 27xx, 30xx, 31xx, 34xx, 35xx, 36xx, 42xx, 44xx, 46xx)
From the Roku home screen, select Settings > System > Factory reset > Factory reset everything, and follow the on-screen instructions.
Classic Roku players (Models N10xx, N11xx, 20xx, 21xx)
From the Roku home screen, select Settings > Factory reset, and follow the on-screen instructions.
Then proceed to the installation of the ROKU.
You will need to go to www.roku.com and create your own account if you don't already have one.