You can cancel within the client area by going to Services > My Services > View Details > Management Actions > Request cancellation.
This will cancel the individual server, not the account with us. If you have more than one server, they will need each to be cancelled individually.
If you're using an automatic PayPal payment subscription, you will have to cancel the payment subscription within your PayPal account, to ensure that you do not continue to send us payments. A guide on cancelling a PayPal payment subscription can be found on the PayPal website.