Log into your account at https://my.roku.com.
Scroll down to the list of your liked devices.
Locate the stolen device and click its "unlink" link.
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