One proactive action a mod could take is to figure out the forum ethics, make it explicit and summarize it in a post, so that people could refer to it and refer others to it. This way if there is an argument, the participants could check their actions against the explicit written norms. In my experience a forum ethics is some combination of consequentialism, deontology and virtue ethics. Some examples from other forums:

• strive to make only positive contribution and leave the comment thread in at least as good a shape as you found it (vaguely consequentialist)
• no spoilers (a common ground rule in fiction discussions)
• no trolling (sort of virtue-based)

Admittedly, this forum has been working reasonably well without explicit guidelines, and a discussion of forum ethics might be a net negative, so maybe one should leave well enough alone.