There are many reasons to cancel your subscription, but the main purpose is to stop renewals. Each subscription is automatically renewed, so you will be automatically billed. By canceling your subscription, you will no longer be billed.
You will continue to receive Premium features until your next bill date.
Canceling is easy. Log into the account you have the subscription you are wanting to cancel. Then go to Curse Premium and click the "Cancel" link to the right of your subscription details.
Follow the instructions from there to complete the process.