What is the ruling of one who says "I am free." when he is advised?

If he said that he is free intending that he is free from enslavement by the creation; then yes! He is free from enslavement by the creation. And as for intending free from servitude to Allah 'Azza wa Jal, then he has indeed erroneously misunderstood servitude to Allah, and he does not know the meaning of freedom. For if he is not subservient to Allah then he is subservient other than Allah. He will only deceive himself if he says; "I am free." meaning by it that he is not required to abide by the obedience of Allah.

What is the ruling of a sinner saying 'I am free to do what I want,' when rebuked?

This is erroneous, we say; you are not free to disobey Allah. Rather, if you were to disobey your Lord, then you would have existed from the servitude which you claim in your worship to Allah into the servitude of the Shaytaan and lowly desires.