As far as I understand, the advocates of theories that negate free will are not basing their 'theories' on philosophical grounds, but on the grounds of physics, chemistry and all other sciences that regards reaction/action in matter/energy.

As such, you in order to prove such a theory right you must fail to prove it wrong. If you want it proven wrong, just prove it wrong but sadly I think that is beyond the scope of philosophical insight, you'll need hard science.

As far as philosophy goes, it is fairly clear to everyone that we seem to possess a free choice of action in our everyday lives and as such, we can probably happily disregard the 'materialist' view, because it doesn't matter for us.