This article how to delete a group and what happens when a group is deleted. It also provides the location of the "Delete" option.

If you’re the group owner or a moderator with the privilege to delete a group, you’ll see a link to delete the group in the lower-right corner of the "Management" page.

To delete a group:

1. Click Delete.
2. Type a reason into the field provided.
3. Click Confirm.
4. Click Confirm again to complete the deletion. 

Deleting a group will remove the group from Yahoo! Groups, unsubscribe all members, and eliminate all archives and shared files/calendars.

Note: Once a group is deleted, it cannot be restored. Please be sure you want to delete the group before proceeding.