You don't.
A workgroup is just a group of computers. It's not stored on any one PC. If you change the name of your workgroup on your PC, you are automatically removed from that particular workgroup.
There's no "removing the workgroup" altogether because there's no place that it's actually stored.
Now if you were on a domain, then there would be one or more computers that control the domain, you can add and remove PCs from the domain, and you can remove the domain altogether.