Bukkit

The anti-spam automatically warns, then bans spammers if they persist. It stops caps spam, copy/paste spam, jibberish spam, macro spam, login spam, and even ASCII art spam (ex. nyan cats). Even if the spammer has access to 10's of accounts all hooked together by a robo-chat app, he'll only succeed in getting all of his accounts banned faster. In practice, many griefers ban themselves, because when they realize they can't grief any builds, they resort to chat spam and catch a ban, even when there aren't any moderators online to ban them. :)

You needn't worry that players who aren't spamming will be banned. Players will only be banned if they're consistently spamming over many messages and ignore the spam warning. Reasons for action taken against spammers will be logged, including all messages sent (even those which were muted).

If you see that a player's message was muted (or the player was banned), and you don't understand from the message content the reason for the action, the answer is probably in the pattern of his previous messages. GP considers repeating oneself or sending messages too close together to be spam (although there is a tolerance for this, since non-spamming players do both on occasion).