Wikipedia article on cherem lists the Talmudic rules on what offenses qualified for niddui.
However, it doesn't seem to actually list anything specific (aside from monetary issues that caused repeated niddui) which would lead to indefinite-period official cherem imposed on a person.
Was/is there a codified set of rules of what does and does not rise to the level of cherem? (specifically, after 600CE)?
Or was it pretty much up to individual Rabbis based on their personal judgement of what was good for their specific community?