Call them out early — If someone is going to debate in bad faith they usually do so from the beginning. I call people out for ad hominem attacks from the get-go, and ask them to reconsider comments and reissue them without calling into question my identity, integrity, or skill as a writer. If they fail to do this on the first time of asking, then I load the snarktillery. If they fail on the second comment then I reserve the absolute right to use my skill with words to eviscerate, maim, and generally create a dystopian clusterfuck around them.