Such a person believes that whatever you did or do has been predetermined by a god or another omnipotent force. It's as if free will didn't exist.
e.g. "It's no use trying to discuss it with you. You are ..........
A fatalist or a predestinarian.
predestinarian: A person who believes in predestination; a fatalist. Also in extended use. [OED]
From a philosophical standpoint:
In philosophy, a fatalist is someone who holds specific beliefs about life, destiny, and the future. Fatalists share the certainty that fate has already been laid out in front of them, and that they have no real control over what will happen.
You can also check:
Calvinism (also called the Reformed tradition, Reformed Christianity or the Reformed faith) is a major branch of Protestantism that follows the theological tradition and forms of Christian practice of John Calvin and other Reformation-era theologians.
Reformed theologians teach that sin so affects human nature that they are unable even to exercise faith in Christ by their own will. While people are said to retain will, in that they willfully sin, they are unable to not sin because of the corruption of their nature due to original sin. To remedy this, Reformed Christians believe that God predestined some people to be saved. This choice by God to save some is held to be unconditional and not based on any characteristic or action on the part of the person chosen.
mechanist: a person who believes in the doctrine of mechanism.
Specifically Anthropic Mechanism - via wikipedia:
The thesis in anthropic mechanism is not that everything can be completely explained in mechanical terms (although some anthropic mechanists may also believe that), but rather that everything about human beings can be completely explained in mechanical terms, as surely as can everything about clockwork or gasoline engines.
...It's as if free will didn't exist.
Ermanen's post gives two great terms.
There's a stronger one that might apply:
Necessitarianism is a metaphysical principle that denies all mere possibility; there is exactly one way for the world to be.
It is the strongest member of a family of principles, including hard determinism, each of which deny free will, reasoning that human actions are predetermined by external or internal antecedents. Necessitarianism is stronger than hard determinism, because even the hard determinist would grant that the causal chain constituting the world might have been different as a whole, even though each member of that series could not have been different, given its antecedent causes.
(From Wikipedia. Emphasis mine)