TrueTwit is a Twitter service which will help you fight spammers. TrueTwit works by sending a DM to each new Twitter follower you have with a verification link where the follower needs to enter the Captcha seen in the image. You will then be notified by email if your new followers pass this verification or you can set TrueTwit to automatically follow them once they have passed the verification.
While in theory, this sounds like an amazing service, it has the downside that it needs to send a direct message to each of your new followers - a behavior which most of the times is considered as spam so people might ignore your message. But TrueTwit might use another method for verifying users:
(although that tweet has been posted on October, 16 - but there is still hope).
Back to TrueTwit - the web app. You'll be glad to know that you can customize TrueTwit notifications, the amount of DM's to send daily and of course, the message text:
Also, if you want stop having to click each and every such validating link from people using TrueTwit, you must also register for this service. You will then pass as a verified user and clicking these validating links and entering the captcha is no longer required for you.