There are a few ways to achieve this. If you simply want to stop receiving reminders, you can delete all your active reminders from your "My Reminders" page. Your account will stay active in case you wish to come back and create a reminder in the future.
If you wish to actually cancel your user account, you may do so in your account settings. Login to your account, click the “Profile” link found under the welcome message. Click Edit under the Personal Information section where you can use the Deactivate Account feature.