This is a famous problem and I was surprised I didn't find it with an appreciable amount of effort. This is to say that I am not taking credit for thinking of it. But I will take credit for putting it on E2.

It starts out with two self-evident (or so I think) truths:

1. The law of the excluded middle. Either proposition P is true or it is not true, there is not a third option. Ie, either there is a god or there is not a god; either there is free will or there is not free will.

2. The law of non-contradiction. That is to say that for proposition P, there is a problem if you say P is true and not true. P and ~P (~means "not") are mutally exclusive, if one then not the other.

Seems simple enough, no? Now make P in the future tense- "I will get laid some time this century." Under normal circumstances this is probably P, but I may die or get into some disfiguing accident (yes, it could even happen to you)... so there is the possiblility of ~P. But only these two posibilities exist. It is not possible for me to get laid yet not, at least once "laid" is clearly defined.

And following from #2, if I do then I don't not. If I don't then I don't do. Follow? Let's say P is correct, then there is nothing that can happen or that I can do to stop it . Even if I wanted to stop it, P is still correct. And the same could be said for ~P.
It is therefor impossible for me to make any decisions that interfere with the fruition (no pun intended) of P. But P can be anything. Let's Make P a choice. "I will clap my hands at 3:00." Assuming my hands are untethered and I know when 3:00 is, P or ~P should be the product of my "volition." But as I said be for, it is impossible for me to make any decisions the interfere with the fruition of P. It follows that it is not my decision whether or not to clap my hands at 3:00, or any other P. And since decision is part of the definition of free will, it therefor does not exist.