Backup functionality has scheduling in two places:

1. In the backups.yaml profiles -> schedule: true/false for every profile.
2. In the scheduler.yaml in default-site-backup task.

There are two issues when trying to disable scheduler by modifying YAML files:
a) If you set schedule: false in backups.yaml this does nothing. Backups are still created.
b) If you remove default-site-backup task from scheduler.yaml completely, the Admin UI says that the task is disabled, but if you run php bin/grav scheduler -j it says the task is enabled and the scheduler still creates a backup.

The only workaround is to set default-site-backup task in scheduler.yaml to disabled.