Trolls should be banned because they disrupt serious discussion. Identifying trolls is often difficult, however. I like to think in terms of "troll score": the higher the troll score, the more likely it is that a person is a troll. Examples of things that affect troll score:
- Having crazy positions (racism, etc.): large increase
- Posting long, well-reasoned posts: large decrease
- High posts-to-time-online ratio: small increase
- Been around for a long time: moderate decrease
- Posting many images: mild increase
- Much discussion of illegal sites: very mild increase
- Requires more moderation than is typical: moderate increase
- Using vulgarity, insults, or memes excessively: mild increase
- Posting topics with undescriptive titles: mild increase
- Seems disdainful of the Bitcoin community in general: mild increase
- Bumping old posts a lot: mild increase
- Giving people bad advice: mild increase
- Poor spelling/grammar: slight increase
- Criticizing moderation: moderate increase
- Complaining about anything: small increase
- Uses ad hominem attacks with unusual frequency: moderate increase
- Lying: moderate increase
- Inconsistent positions: moderate increase
- Posting the same arguments over and over again without bothering to respond to arguments against them: moderate increase
- Posting topics that will obviously evoke emotional responses: slight increase
- Posting replies that are themselves on-topic but which will obviously tend to bring the discussion off-topic: moderate increase
- Bumping old threads with replies that are not really worthwhile: moderate increase

None of these things alone warrant a warning, but when they occur habitually or when someone does many of them, they indicate that the poster is a troll.

Warn people who are doing many trollish things. Tell me if that doesn't help.