Rafael, this is an excellent question, and philosophers do not all answer it in the same way. My own view is that predictability in itself is not a special threat to free will. Suppose that I have free will. Now suppose that you know me so well that you can predict every move I will make. So long as you don't use that information to influence me, but just know it, I don't think that takes away my free will. You are not interfering.

One of the things that makes this question difficult is that we have trouble seeing how free will is ever possible, prediction or not. The classic dilemma is this. Either everything has a cause or not. If everything does have a cause, then it looks like you have no free will, because the chain of causes leading to your actions began before you were born. And if not everything has a cause, if in particular some of your actions are uncaused, then that doesn't seem like free will either. It seems just like a random event. In short, either determinism is true or it isn't; if determinism is true you have no free will; if determinism is not true you have no free will; so you just don't have free will. Philosophers have worked long and hard to try to show what is wrong with this argument, but it's a tough nut to crack.