Login to your online Roku account (https://my.roku.com/account), and under Manage Account you will see "Deactivate Account". May be a good idea to cancel any subscriptions linked to that Roku account as well.
You can also choose to just "Unlink" the stolen RokuTV from your account so you don't lose anything else associated with your current Roku. If the thieves do not know your Roku account password, once unlinked, they will be forced to do a factory reset and setup an account to get streaming capability again.
Not sure what you mean that you don't have internet connection. You can use your cell phone if you have to. I am assuming you mean that the tv is not currently connected to the internet, but a little unclear.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102
Does the Roku device net to be plugged in to deactivate it from my account. I have one I am trying to give to a friend. When I set it up on his TV (not a Smart TV) all my apps and data come up. When I go to my account to deactivate that device, it does not show on my account. Do not know what to do from here.