You can revoke access to the Health app using Apple's Health app on your mobile device.
To disable access for Omada,
- Open Apple's Health app.
- Press the tab labeled "Sources".
- Choose "Omada" from the list of apps with HealthKit access.
- A menu will be presented allowing you to disable access to steps data.
Once this is completed, you may connect an activity tracking device like Fitbit or Garmin to Omada or enter your steps manually from another device.