Can I cancel at any time?

Yes you can.

The cancellations are handled by CCBill or Epoch (whoever you used when you joined up).
You are billed monthly or yearly, so the subscription will still be active until the billing cycle is up.
Please note there is a processing timeline so if you cancel a few days prior to your next billing date, you may get billed for the next month.

If you joined with Epoch:

If you joined with CCBill: