First open settings in the top-right menu:
Next click the "On" toggle on the right to switch it to off:
You can re-enable sync at any time, and all of your tasks will sync up among your devices! You may experience some strangeness with recurring tasks and day notes (and similar) since Marvin will have to choose (seemingly at random) which version to keep.
If you want to keep separate tasks on each of your devices and not sync on any of them, you'll have to disable cloud sync individually on each device.