Once you're logged in at the agency level, you can retire a sub-account, which will remove them from your agency account and billing -- you can find the step-by-step instructions in THIS Resource Center article. Once the sub-account is retired, you will no longer have access to that data. However, if you did need anything after a sub is cancelled, the Support Team can access the information for 30 days, after which the data will be completely purged from our servers.
This means that you'll want to export any contact lists and response data as well as delete signup forms and turn off workflows before retiring that sub-account. If you want to save your sent campaigns for your archives, you can save the webview as a PDF, and you can find more information on that BY CLICKING HERE.
Just let me know if you have any follow-up questions on this, Jasmine!