I take it that the parent is not a registered leader of any type? Parents have irrevocable Full Control permissions to their scout, so if they are any kind of registered leader in the unit, there’s no programmatic way to prevent them from doing this.
If they are not a leader in the unit, then try resetting your Unit Admin position by going to My Dashboard → Administration → My Units → Unit Roster → Your Name → Your unit admin position.
Toggle the approved radio button off then back on without saving, then save. This (usually) resets the unit admin role, but is path dependent.
Then, try changing the Leader position toggle to off for the parent.