You can only delete a workspace if you are one of the owners of that workspace.
To delete a workspace, click on the dropdown arrow next to the name of the workspace, then select the Delete workspace option (which has a trash icon next to it).
A warning dialog will pop up asking you to confirm that you would like to delete the workspace. Press the red Delete workspace button to delete the workspace.
For about seven seconds after your workspace has been deleted, there will be a small popup at the bottom of the screen saying "Workspace deleted" with an undo option. If you click this undo option, your workspace will be restored.