How can I stop verification emails being sent to customers?

Please navigate to Admin -> Workflows -> Email notifications -> Requester notifications -> User Activation email and toggle this off so that you could not automatically send this email to your customers. 

Kindly note that you would be able to send activation emails from the customers' tab where you could choose the unverified contacts from the "showing" drop-down and send them by checking the boxes next to the contacts manually so that they could create an account with your portal.