Here are the steps on how to unenroll the console. (Even if it was set up on a different account)
1) Download the Xbox Insider Hub app on the console. (If it's not already installed)
2) Launch the Xbox Insider Hub. (Accept the TOU if prompted, doing so will not sign you up for any Preview)
3) Navigate within the Xbox Insider Hub to Settings / Manage Devices.
4) Select the current console. (This will always be marked “(Current)” and will appear at the top of the list)
5) Hit the ‘Done’ button.
That should work for you.
Best of luck.
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