How can I deny someone following me on Twitter?

[Dragonus]

I just signed up and got a follower who has been on Twitter for over 6 months with only 3 tweets, two of which are ads for his website (which has pop up ads). It lists him as my follower and never gave me the chance to approve or deny him, and I can't find where to go to delete him.
That worked, but I though it was supposed to ask for me to accept or deny a potential follower? It has now happened twice with no message or request.
If I have to make tweets private, I will leave it as it is. I want others to be able to read them, just wanted to be allowed to accept or deny followers. That's how it should be.