What's the most effective spam filtering tool for reducing moderator workload on a busy community forum?

The most effective tool is usually a layered anti-spam system, not a single filter.

If you want the biggest reduction in moderator workload on a busy forum, the best practical choice is typically:

A modern forum spam filter with reputation + automated moderation

Examples of what to look for:

• Akismet-style spam scoring
• Rate limiting / flood control
• New-user restrictions
  • no links for first posts
  • email/domain limits
  • post approval until trust is earned
• CAPTCHA / bot detection
• Keyword and link pattern detection
• Trust levels / user reputation
• Automatic quarantine or hide-on-detection
• IP / device / ASN blacklisting
• Integration with moderator actions so one-click bans and spam cleanup are easy

If you want one “best” answer:

For most busy communities, Akismet combined with trust levels and rate limits is one of the most effective setups because it catches a lot of obvious spam automatically without requiring constant manual review.

Best choice depends on your forum platform:

• Discourse: built-in trust system + Akismet + rate limits is excellent
• phpBB / XenForo / vBulletin: their spam plugins plus manual approval for new users
• Custom forum: use a spam detection API plus your own throttling/approval rules

What reduces moderator workload the most

1. Stop spam before posting with trust rules and rate limits
2. Auto-classify likely spam into a review queue
3. Let moderators mass-remove and ban with one click
4. Train the system from moderator decisions

If you tell me what forum software you use, I can recommend the most effective specific tool or plugin for that platform.