So are you suggesting that the statement “Santa Claus does not exist” requires the existence of Santa Claus to be true? (Of course not, I’m just making a point.)

However, I think what you’re suggesting is this: to state that Santa Claus does not exist we have to in some way define Santa Claus. So in that sense, I suppose that something of Santa Claus exists – namely, his definition.. But the definition of Santa Claus is a thing separate from the hypothetical being.

To deny the existence of something requires us to define or at least label that which is being denied. And although a definition of a thing is itself a thing, it is not the same thing as that which it defines. (I hope that makes sense!)

So back on topic, to deny the existence of free will requires us to define free will. The definition of free will exists, but free will does not.