It's a pity that you have decided to cancel your Instories subscription. However, if your decision is final, then follow the instructions:
Go to the Members section in the menu on the left of the screen.
Tap on the 3 dots icon near a team member email to open the menu.
Tap on "Revoke access to Pro".
Do the same for all active team members.
Now your subscription is cancelled and won't renew anymore.