You cannot cancel a domain since it remains registered until it expires. However you can turn off auto-renew for a domain so that you won't be automatically invoiced for it when it nears expiry.
To turn off auto-renew for a domain log into this support portal, choose Domains --> My Domains from the menu, you should see a list of all your domains. Click the domain want to disable, then click the Disable Auto-Renew button.
Once this is turned off you will no longer receive new invoices for this domain but you will still receive reminders for any invoices already generated. If this happens then just email us with the invoice number and we will cancel the invoice for you.
There is no need to disable auto-renew on an expired domain, once a domain has expired you should not receive any new invoices for it.
You should also turn off auto-renew on a domain if you transfer the domain away from us without cancelling your account, we are often not notified when a domain has been transferred away and may mistakenly continue to bill for it.