I don’t think you can take away free will, it’s one of the main rules and central point of this planet’s purpose.
Mind control works via either persuasion, confusion, or breaking someone’s mind so badly they can’t think their way out of it.
In the first case, they keep their intelligence - they may they think it’s their idea, or may be acting to avoid something worse they’ve been persuaded is real. With confusion they make incorrect decisions because they don’t have all the information ad keep trying to place incoherent input into a known paradigm. In the latter, they are quite mad, may have lost major parts of their soul to protect it, and will be unable to fully perceive reality in a functional way.