For the cancelled accounts we can send the case to our cancellation or delete company Queue, you just need to contact us and request to escalate the company you want to delete, we may ask extra information to verify the account and to check if the right user is requesting it.
You can still sign in with the old credentials you have in purchasing accounts just click on sign in - directly and it will route you to a sign in page.

If you want to reactivate the account and just want to start new- you can also use the purge company file option to start over. 
https://quickbooks.intuit.com/learn-support/en-us/help-article/intuit-subscriptions/delete-data-star...