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Dear Yehuda Yatskan,
Good day! Thank you for posting to Microsoft Community.
As per your description, it seems that you want to hide or change the "Unassigned items" bucket in a SharePoint board view list.
If my understanding is right, we've done some research and tests on our side, we're afraid that there's no out of box way to remove or edit unassigned bucket in the first place of the SharePoint online board view list currently. We've found that there is some feedback for the ability to modify a Board view has been marked as thinking about by Microsoft product team based on our research.
Personally, I completely agree with you, this is indeed a good addition to the current features of List board view. It would be definitely ideal if we could remove or edit unassigned bucket in the board view. Given this situation, we sincerely recommend you vote for this similar idea in SharePoint Feedback Community:
Remove "unassigned" bucket from a List's Board view · Community (microsoft.com)
Product Team is in-charge of the feedback site, and they are constantly checking the comments and feedback from our customer. Many features have been developed or improved based on users' comment.
Thank you for your cooperation and understanding!
Sincerely,
Rhoda | Microsoft Community Moderator