Do heroin addicts have free will? I disagree with Sam Harris on many topics, but on free will, he used an illustration involving a mass shooting in which the shooter was afflicted by a tumour. The tumour placed pressure on his brain in such a way as to alter his disposition, and he became violently aggressive toward his family and eventually his society before shooting several people and then himself.
I don't remember whether this was a fictitious example or a true story, but either way, it's possible for this to happen. You can force someone to experience certain sensory perceptions by manipulating portions of their brain. You can certainly change someone's disposition through torture, medication, indoctrination, etc.
I don't believe we would respond correctly to environmental stimuli, and by correct I mean conducive to survival, if we had free will.