The public code in news.arc (which doesn't the anti-abuse code) suggests:

  * The test for a comment being auto-killed (marked [dead]) is "(or (ignored user) (< (karma user) comment-threshold*))"
  * comment-threshold* = -20
  * Karma is still updated on item authors if their post is dead - as long as the tests on the person doing the voting pass.
  * Once you have the ignore flag set, only administrators can remove it - it is independent of karma.
  * You get the ignore flag set for posting a link to a banned site, posting a comment containing banned text, or if one of your posts is blasted or nuked by an administrator.