Did you subscribe through Hayu direct or through NOW ?
What devices was you using when watching Hayu, was you watching Hayu through the NOW Stick or NOW box or a device through the NOW App ?
If using through NOW, then open the NOW App and click on the gear wheel icon (top right of home screen) and see what NOW login credentials you are using, then login to the NOW website with those details and under My Membership see if there is a cancel button option next to Hayu.
If you subscribe to Hayu through Hayu direct or Amazon for example, then you need to go into those accounts to cancel your Hayu Membership.
I don't know if there is anything in this linked article below helps.