Setting > System > About will show you the email address associated with the Roku account to which the device is linked.
Log into your account at https://my.roku.com, using that email address.
Scroll down to the list of linked devices. You can unlink any unrecognized device there.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X
