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. The "Cancel" link only appears if you are subscribed to a plan.
Once you've clicked the "Cancel" button you'll be directed to a cancel page. Follow the steps on the cancel page.
If you cancel your paid subscription in the middle of a subscription period (which is one month for monthly plans, 12 months for annual plans) your account will be transformed into a pre-paid account and your subscription will continue to function as a paid subscription until the period ends. At that time, your paid subscription will be canceled and your account will be downgraded to a Hobby Plan. You will no longer be billed for anything unless you decide to re-upgrade your account again in the future.