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.

[Gerben Oolbekkink]

visit his profile on twitter and click the button in the shape of a head next to the follow button

Click block user.

If you think he is a spammer also click report as spam

This will remove this user from your followers

[Marc Finnin]

You haven't put your account on private. Private accounts has to accept or deny followers and cannot be retweeted. To put your account on private, go to settings, then scroll down to Tweet Privacy, check the box next to protect your tweets, click save, then it will ask you to enter your password. Then your account will be private.