I would make a rule: any discipline against a troll will be matched by identical discipline against anyone who engages that troll, even in attack.

In practice, such rules create echo chambers where any discussion that the moderators don't agree with is silenced. It's theoretically possible for a human being to be utterly impartial when deciding whether conversations are useful, but it requires near-superhuman patience and tolerance.

Quite a lot of the problem is that the category of 'troll' quickly expanded beyond its original meaning; in everyday Netspeak, it generally refers to "a person who persists in saying things I don't wish to tolerate".