A: You have options!
First, you should know that you can unfollow anyone you want, at anytime. (It's Twitter policy!) Twitter explains how to that here. And Twitter is very proactive in zapping spam accounts. (You can report spam here.) Of course, if your account is public, this will not prevent anyone from following you or reading your tweets.
The most effective way to prevent someone from reading your tweets, and following you, while keeping your account public, is to block the user. Twitter explains:
We provide all Twitter users with the ability to block other users. Blocking prevents a user from following you, sending you an @reply or @mention, or putting your account on any of their lists.Twitter explains how to block users here.
Finally, you can lock down your account and simply NOT allow any unapproved user to read your tweets. (But if you ever go public, any user WILL be able to read your tweets.) This is what I do! I approve each user. I can follow someone without allowing the user to follow me. (Usually I allow someone to read and do not follow. Also an option!) You can adjust this in your settings.
I think this covers all the bases. Questions?
Following Rules and Best Practices, Twitter.com
Unfollowing from your following page, Twitter.com
How To Block Users on Twitter, Twitter.com
How To Report Spam on Twitter, Twitter.com
*This question has been abridged.