There are two types of accounts within the DashClicks platform:
Main accounts (Your account type)
Sub-accounts (A client's account)
While you can control some aspects of what happens within a client's sub-account, that account still ultimately belongs to that user.
For this reason, you cannot delete a sub-account after the user accepts the invitation to DashClicks. Their account will always be theirs to use from that point forward.
Likewise, the contact record you created to send the invite is automatically bound to the sub-account once the invite is accepted. For that reason, you also cannot delete a contact record once this association is formed.
If you wish to prevent the creation of unwanted sub-accounts, be sure to cancel invitations as soon as possible. You can learn how to do this here.