In order to remove an Organization from Exchange 2010 multi-tenant you must first delete all mailboxes from the organization. You can delete most of them through the ECP (Exchange Control Panel), but you will still be left with the administrator mailbox you are using, DiscoverySearchMailbox, and some others.

First you must find your organization you want to remove. You can do this by typing:

• Get-Organization

Next you want to remove all mailboxes from the organization. Like I said above you can delete most of them using ECP or you can remove all the mailboxes quickly by typing:

• Get-Mailbox-Organization"<ORG NAME HERE>"| Remove-Mailbox

Now that all mailboxes are removed you can type this to remove the organization: 

• Remove-Organization -Identity "<ORG NAME HERE>"

Give it a minute and you should be able to do another Get-Organization and notice that your organization is gone! I took a picture for your viewing pleasure: