I think circular logic is fairly well defined (from Wikipedia, “Circular reasoning is a logical fallacy in which the reasoner begins with what they are trying to end with. Circular reasoning is not a formal logical fallacy, but a pragmatic defect in an argument whereby the premises are just as much in need of proof or evidence as the conclusion.”)

That said, yes it’s not grounds for a ban. I wouldn’t block over it either unless the person in question is being obnoxious and e.g. following me around between threads and trying to stir up argument about the subject of contention or resorts to personal insults or something like that.

You will never have an agreement on it. Some see statements as axioms and others do not. Without premises, every form of reasoning can be regarded as circular.

You can have an authority on it that determines something as circular or not. But then you will inevitably end up with dogmatism.

Someone once said that circular reasoning works if the circle is large enough. Knowing the fallacy might help you detect faulty reasoning in your own thoughts. It doesn't allow for much more practical applications.