From the Admin level, click "List Resellers" then click the "Select" link at the top right. This will select all resellers on that page. Click the Delete button, then confirm to finalize it.
The user accounts under each reseller will remain, IIRC. I believe that if you want to delete the user accounts as well, you should login to the reseller's account, then select all user accounts for deletion before deleting the reseller. If you've already deleted the resellers but now you want to kill off the user accounts, you can use the "Show all users" link in the Admin level. If you click the "Creator" link at the top, it will organize the list by reseller, then you'd have to tick each user you want to delete.
