The Twitter Blacklist has a wonderfully simple API with a yes/no response. Just call http://twitterblacklist.com/api?username where ‘username’ is the id of the suspected spammer. If the response is ‘yes’, it’s a spammer.
Update:
There is already a very cool Greasemonkey script (created by @chimeric) that uses Twitter Blacklist to alert you when you view a spammer’s profile page. Look out spammers!
