With their old control panel, the automatic renewal option was on the Billing Management page. I'm not sure if this is still the case since they changed the look of the control panel and I have already have auto-renewal turned off on my subscription. I usually manually renew on Black Friday when they run their promotion.
Going by their support page, it appears to be the same with their new layout, i.e. log into the control panel at https://vpn.hidemyass.com/ and then click the 'Manage' button next to the subscription. Hopefully there should be a button "Stop Renewals" on this page.
If your subscription says "Non-Renewing", this means it will not auto-renew.