I'm having a hard time articulating exactly what the main issue with this argument is. It's got that "It's so wrong it's not even incorrect" flavor to it that makes addressing it difficult in a weird way.
The root, I think, is that the people who make these kinds of objections is that their concept of God, particularly God as we Christians know him, comes from cultural osmosis rather than from an actual study of how God presents Himself to us in the scriptures. Of course not every unbeliever, or really any unbeliever should be expected to be biblically literate but because of the place Christianity has in the culture and history of western civilization people make all kinds of assumptions about what they "know" Christians believe or what they "know" is in the bible and the creates it's own unique problem when trying to deal with surface level arguments like these.
It doesn't help that the major branches of the faith actually differ on the extant of how free man's will actually is. Being staunchly Reformed in my theology I would say that our will isn't free. This doesn't mean we are robots doing as we were programmed only that we are not entirely autonomous. The only being whose will actually is completely and totally free is God because being uncreated and self-existent the freedom of His will sets the maximum amount that any will can be - that is God cannot create a will freer than His own and no will can be as free as His since all other wills being made by Him, no matter how free, are at their foundation dependent on His will to even exist.
Biblical prophecy is incoherent if our wills are completely free because events in history occur as a result of countless decisions, emotions, actions and reactions on the parts of millions upon millions of people. If God knows the future, indeed if what the scriptures say is true and God established the beginning and set it apart from the future and ordered time according to His purposes than none of our wills in History can overthrow what God has ordained.
And yet, as we see in the case of Joseph when his brothers sell him into bondage, after they go into Egypt to seek aid during a famine, Joseph remarks to his brothers that though what they had intended in their hearts was evil, what God intended to accomplish through their actions was good so the will of man is free to depart from the will of God in it's intentions it nevertheless leads man to accomplish the will of God anyway. We can go further into this and look in Pauls epistles where he describes how we are dead and in bondage to sin and our wills are to do evil so long as we are in Adam and only after we are in Christ we are able to do what is pleasing to God.
So there is a degree to which our wills are free but only in as much as they are bound to obey our sinful natures and can never be so free as to interfere in the freedom of God's will.