SpamGuard is a fairly mature Bukkit Server plugin that is designed to prevent spam bots and annoying players from sending messages over and over.
Featuring IP-banning, tons of configurable options, command spam prevention, word blacklisting, repetition detection, capslock blocking and more, SpamGuard is the leading anti-spam plugin on Bukkit!

Downloads: 50,000+ (Current as of August, 2012)

Subpages (1): SpamGuard Wiki