Open Animal Crossing: New Horizons and select the user with the Nintendo Account that originally enabled island backup.
In order to disable island backup:
Your Nintendo Switch system must have a stable connection to the internet.
Your game must be updated to the latest software version (1.4.0 or later).
From the title screen, press the – Button to access the Settings menu.
Select Disable island backup, then Yes, disable backup to confirm.
If island backup cannot be disabled, it may be because your Nintendo Switch is not currently connected to the internet or you are not using the same Nintendo Account that was used to enable island backup.
After island backup is disabled, it will not be possible to restore island data in the case your Nintendo Switch system is lost or stolen. You can re-enable island backup using a Nintendo Account that has a resident on the island and an active Nintendo Switch Online membership.