Only the global administrator (system-defined role) or their equivalent (custom defined role) has the necessary permissions to delete a User Group. Once a user group is deleted, it will no longer exist. To delete a User Group, follow the procedure given below:

Procedure: 

  1. Go to Manage → User Groups → Select the User Group you want to delete.
  2. Click on Delete button in the Edit User Group window.
  3. Click on Yes to confirm.

In case you are unsure, it is better to disable a User Group rather than deleting it.  A disabled User Group will be still visible and can be activated again.