I guess you probably want to say “no way of contact”...Anyway, in this case you can opt certain steps for account verification such as verify the information of client through phone before account set up. In case of more doubts, you can ask them for a scan copy of passport or driving license; match the details with order form.
However, I would strongly recommend you to offer a “bank transfer” option for those clients whose payment accounts are different from the WHMCS account.