A workspace must first be archived before it can be deleted.
If your workspace is already archived, follow the instructions starting at number 4.
How to delete a workspace:
Select My workspaces from the "Me" menu
Find the workspace you want to delete from the list of open workspaces
Click the "..." menu on the right side and select "Archive workspace"
Open the "Archived" tab from the top to view all archived workspaces
Find the workspace from the list of archived workspaces
Click the "..." menu on the right side and select "Delete workspace"
Deleted workspaces cannot be restored, so think carefully about whether the workspace needs to be deleted or it is enough just to archive it.