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You can cancel your paid subscription on AddEvent.com in the Membership and Billing section of your account. 
In the section, you can see your current plan. Next to the plan information, there's a Cancel link. Cancel only appears if you are subscribed to a plan.

Once you've clicked Cancel, you'll be directed to a cancel page. Complete all the steps on the cancel page. If you cancel your paid subscription in the middle of a subscription period (one month fo...