A website has launched which claims to be able to identify spammers which follow you on Twitter. Using an algorithm which includes followers, spammy words and how the person tweets, among other things, Twitblock allocates points as to whether the profile is likely to be a spammer or not.
A Twitter username and password are required to log into the site, and when you're in it scans through your followers list. Any suspect profiles are listed at the top, along with the option to block the spammer with a button click.
Of course, unless you're following the spammer back, you won't see any of its spam in your Twitter stream, but this enables you to totally block a profile from seeing anything that you send out, too.
If you have a spam problem on Twitter - perhaps you legitimately tweet about Viagra all the time? - then you might want to give TwitBlock a go.
Update: To clarify, TwitBlock uses OAuth via Twitter, so doesn't store your password.