There seems to be some confusion about spam. If a spammer follows you, it really does not matter all that much. It is when you follow a spammer that you will be subjected to their content-until you unfollow. Spam accounts following you are not gathering information in that way. If they want to gather information, they use Twitter search and even an account you blocked can find information about you and anyone else there. The only information that is not available in Twitter search is protected updates and those who are “gone from search”. (See article) Even an account that you blocked can go to your page and read your updates, unless they are “protected”, but protecting your updates seriously limits your ability to gain followers, so what to do?

http://modelsupplies.wordpress.com/2009/09/16/how-to-avoid-spam-on-twitter-follow-facts/