This is the typical way to delete a usergroup from APM CE:
To delete a user group:
But the approach only allows deleting 25 users (on one page) at a time. Is there a way to do a batch delete of a large group of users (Such as 275,000)?
By design, only 25 users or usergroups can be deleted at a time,. However, one can customize the UI pages to allow deleting more users or usergroups at a time.
Here are the instructions for deleting 1000 (or any number) of users.