If you initially purchased iRacing directly from www.iracing.com with a Mastercard, Visa, American Express, Discover or Paypal, you can cancel your membership by logging into http://members.iracing.com and click on My Info.
Or you can log into your account directly here: https://members.iracing.com/membersite/account/Home.do
If it then says "Turn ON auto-renew" - you have successfully turned off auto renew.
In addition, ACTIVE will change to DON'T RENEW.
This will turn off the auto renew feature and your credit card or PayPal account will not be charged the next time your subscription expires.
If you purchased iRacing from Steam, you will need to contact Steam Support as iRacing and Steam have two separate purchasing processes.