Twitter is all over the place and so are the spammers. Recently I and very possibly same as all of you may have seen in increase in number of people following each other and so are spammers. It's pretty easy to search some one and with a simple click of button begin to follow.
One of the good things is that Twitter sends out an email for every follower that follows you. So you can go back and check who is following up. If the person is a spammer or some one that does not fit your circle, you can report that user as spam.
An an example I got the following email by Twitter reporting some is following me as shown below:
As clearly you can see, no one is following this person and this person is following so many. Likewise like at the profile statement and link its a spammer. At the bottom of the email highlighted in green rectangle , there is a link in the email which you can click and report back to Twitter about this.
On clicking the link, it would launch Twitter website and Twitter would mark this account as spam and the person would be disallowed from following you. What I don't know for sure if that blocks this account from following other accounts or every one has to mark it spam for them self, my guess it is the latter. But anyways it will work for you at least, Twitter can figure out the rest.
The following screen shot confirm the block.