Hi Philip - welcome to the community!
You can turn members on single boards back to a guests by going to 'Members' in the Workspace menu on the left.
This will show you a list of your Workspace members, and next to each member select 'Remove' to remove them from the Workspace and turn them into guests.
If you're on the free or Standard plan, by default, removed members will remain on any boards they're assigned to.
In the Premium plan, when removing a member, you'll be prompted to confirm whether you would like to keep them assigned to any existing boards or remove them from all boards.
We have a guide available with more information about removing Workspace members here: https://support.atlassian.com/trello/docs/removing-people-from-a-workspace/
I hope that helps!
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