Domain names cannot be cancelled, but you can remove the auto renewal so you will not receive future invoices for the domain.
Log in to the portal.
Click on My Domains
Click on the icon to the right of the domain you do not wish to renew
Scroll down and uncheck Auto Renew
Save your changes
You will not continue to receive any more renewal invoices for this domain. However, if an invoice has already been generated, but not paid, you must enter a ticket to have that invoice cancelled. Any paid domain invoices are nonrefundable.