I believe you can cancel on the ESO website (this site) regardless of how you signed up. Just go to Account (top left), sign in, then Manage Membership, Cancel Membership.
PC | NA
Guildmaster of Noob Nation
Snegurd Limp-Spear: Nord DK tank Milamber of Midkemia: Breton Sorc dps
Depends how you enabled it. If you're playing the game via Steam you need to do it under your account management via Steam. If you use the PC client from the site, you do it on the site in your account.