Rokus are complicated.
The way they work is a bit different than other Plex clients.
First, the Roku device is signed into the Roku user account of its owner.
Then, this Roku account is linked to the Plex account.
You should monitor the Plex Dashboard. When the Roku device is playing something from your server, you should be able to see with which Plex user account it has been linked.
Is it your own plex user account or the account of a friend plex user?
If the Roku device was linked to your own account, you can sever this link on this page, at the bottom: Sign In | Plex
However, if you yourself do also own Roku devices
and the Roku device in question was signed into your own Roku account
then getting rid of it will only be possible by removing it through the Roku website.
