Give them free reign until they hang themselves, for people in a position of power.
Give them free rein for pets and kids in the house.
Language is one area where reality actually is determined by consensus. Prescriptivism always loses to descriptivism in the end.
I can’t make up my mind about it – intuitively, I would have said ‘free reign’, as meaning ‘given sole authority over one’s actions’, but also reins-free, as in ‘I let the horse go reins-free’, meaning something like ‘without any external coercion’. Though googling indicates I may have made up that expression entirely, since it doesn’t seem to be used anywhere…
I would normally write it as ‘free reign’, but that’s because my spelling sucks - I think of it as rein, my fingers just insist on inserting the g.