> Android could have, for example, a 24 hour "cooling off" period for sideloading approval.

And, to prevent the scammer from simply calling back once the 24 hours are gone, make it show a couple of warnings (at random times so they can't be predicted by the scammer) explaining the issue, with rejecting these warnings making the cooling off timer reset (so a new attempt to enable would need another full 24 hours).