Which of the following is correct?
I NEVER called him a stupid person, or
I DIDN'T call him a stupid person.

Thank you.

I think they are both correct.

The first sentence denotes a degree of finality. I would think that you have never called the person stupid, and don't ever intend to. In which case I would re-phrase the sentence 'I have never called him a stupid person.' to avoid sounding child-like.

The second sentence gives the sense of referring to a specific instance, and is fine.

Thank you both.