Even if the account in question has been removed from all cases, the error states "Account cannot be deleted. This account is a custodian of a case."
This is by design, and is a legal security measure. Any account that has been a custodian at any point in time to any case cannot be deleted in the Veritas Alta Archiving system.
While these accounts cannot be deleted, they can be renamed, disabled from various services/functions within Veritas Alta Archiving.
For assistance with accounts please contact Technical Support.