One of the best things is done in advance: make sure you have a really solid set of rules and guidelines. It makes moderators jobs much much easier if you can clearly cite where a rule was broken — and users are much more amenable if they don’t feel personally targeted.
Beyond clear rule violations, I tend to favor a light touch. Splitting topics is invaluable, because it doesn’t feel like censorship but you can punt them to a category like #off-topic or even unlist it. This can allow the conversation to resolve instead of festering and popping back up in new topics.