Hello Tim,
Is your Tenant used for development of customer specific solution? In case it is, then that could be the reason why you are unable to terminate the old one.
Please refer to the Documents and information on the following page via the Business Centre https://wiki.sme.sap.com/wiki/pages/viewpage.action?title=New+Tenant+Processes+1308&spaceKey=OnDemandWiki in case you have not already consulted it,
It explains the System Provisioning since Release 1308 in further detail.
Hope this helps.
Regards
Nick