When workspaces are no longer relevant, they can be deleted by a workspace admin or a Subscription Owner.
You can access your current workspace's preferences by typing Ctrl-Shift-W.
From here, click the red deletion link at the bottom of the dialog. This will take you to a confirmation form where you must enter your password before clicking "Delete Workspace". Deleted workspaces cannot be recovered so be absolutely sure you want to delete them before doing so.