I am an atheist when it comes to the supposed existence of an all-powerful Flying Spaghetti Monster that created everything.
It's true that the existence of FSM is something I can neither prove nor disprove 100%. After all, I am not omniscient. I don't know everything there is to know about everything. But I think I am safe in denying the existence of such a being.
For one thing, there is a lack of credible eyewitnesses. If any person has ever claimed to have seen FSM -- and I'm not aware of a single one -- that claim was never corroborated by anyone else.
There are also no authoritative written accounts of FSM making itself known to its creation, and nothing chronicling FSM's work throughout history backed up by thousands of ancient manuscripts. In fact, most of what has been written about FSM has been written by admitted nonbelievers within the last five years.
Furthermore, I know that spaghetti is a physical creation, and as such it cannot exist outside of the material realm. To conclude otherwise goes against all logic and reason.
(By the way, I would use the same arguments against the existence of the Invisible Pink Unicorn.)