How about something not on fire? That would be impossible to put out.

There are other choices. Experiment around; maybe find something water-reactive

Iron oxide isn't an oxidiser. it's the oxidising agent in the goldschmitt reaction; but not an outright oxidiser.

But the fact that it was already put out, and thus causes no more destructive nature than it usually would defeats that doesn't it?

Technically to "put out" I think is an action to put something in the state of being "out," and as such there is no need to put something out that is already out.

Also, isn't nitric acid a somewhat strong oxidizer?

Dog was obviosly joking but you were well put I admit....

And yes, nitric acid is a strong oxidizer.... but there are stronger and safer!!!

Nitric acid is also an aqueous solution... Good luck setting it on fire.

damnitdamnitdamnitdamnitdamnitdamnit I knew that

well...i just said it was an oxidizer.

what about bleach--powderstyle.

It is an oxidizer, but very,very weak...
electrolyzing and adding potassium chlorate (salt substitute) gives you the strong one: potassium chlorate

And an irrelevant p.s. (Ghost, if I could by an activated carbon mask I would. That's why I am asking.)

I think you mean adding potassium chloride

potassium chlorate is an explosive right?

No, its an oxidizer