To manage the workspaces that external members belong to, follow these simple steps as an admin:
Click on your avatar located in the top right corner, then select Manage my organization from the dropdown menu.
In the left navigation panel, click on the Members link.
Hover over the external member's name, then click on the drop-down menu that appears. Select Manage workspaces.
Hover over the workspaces from which you want to remove the external member, and click on the "X" button that appears.
Note: The warning icon for an external member indicates that they exceed the two workspaces limit. To resolve this, either remove them from some workspaces or modify their role to "Participant".