Delete his browser in the Devices tab in the admin panel
Jellyfin: The Free Software Media System
Current stable release: 10.10.1
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I don't know that the user can do it, or at least I can't find it. But if you (presumably the admin) goes to the admin dashboard, then devices, and delete the device in question, that should solve your problem. It's only labeled as browser:user though, so if your user has a lot of devices it might be laborious. Just tested on my local instance and it immediately kicked the device.
Thanks a lot! I'll do just that. The documentation isn't all that clear what deleting a device implies. EDIT: Interestingly, shortly after deleting HIS device, my own Jellyfin media player prompted me for a login on my desktop. Hmmm. Curious.