If you are the Owner of a Workspace that you want to close, you can easily disable your Workspace by going to the settings gear wheel and then Workspace Settings> General Settings.
Once there, scroll down to Disable Workspace and click on Go to the [Workspace Name] Organization Dashboard.
Find the name of your Workspace, and click on the trashcan icon next to it (on hover, it will say Disable this Workspace). Once it’s disabled, the Invite button will change to Disabled and turn grey, and you’ll see a green confirmation notification.
You can re-enable a disabled Workspace by clicking the arrow icon next to the disabled Workspace. The Invite button will turn active and white. If the Workspace is part of a disabled Organization, you'll need to re-enable the Organization first (and all of it's Workspaces).