The requirement is only that you contact the customer. It doesn't specify that the contact must be in writing. That said, use of email rather than postal mail would not require E-SIGN notice/consent and opt-in to allow for electronic notice. You may, as a matter of risk management, still choose to require E-SIGN disclosure and opt-in before using this method.
Technically, there is no requirement by regulation to contact the customer at all. You could just immediately eliminate their ability to conduct such transactions in the future or transfer them to a more appropriate account type. They were notified of the account restrictions at the time they opened the account.