It costs you nothing to be followed and to prevent them from following you, you need to block them or go private. I can't believe many people will go through your list of followers to see who is following you, especially if you have over 100 followers.

There are followers, and there are spammers. I set Twitter to send me an email everytime some one new follows me, and I take the 30 seconds to look at the new guy's stats, and scan the profile. If it's obvious spam, I block the account. If it's really interesting and useful, I follow back. Otherwise, I let them follow. The internet is a big exchange of ideas, and why should I censor who sees my ideas, as long as they're not borrowing my status to boost a scam?

I usually will spend no longer than 5 minutes looking through my followers and seeing if they are worthwhile or not. I will definitely get rid of them if they look like spammers.

I am actually working on a program to evaluate followers to determine if it seems like a follower I would like to follow back or someone I would like to block. Quite possibly, I may turn this into a hosted solution. Send me your twitter name and I will ping you when its ready. Might be a solution to your problem.

But to answer your question, I often find myself evaluating my followers and if they seem the least bit 'sketchy' I block + report as spam. Especially if I seen any sort of advertisement in their tweet stream, so if you use Ad.ly then please don't follow me. On another note, I am very selective with whom I follow, I usually evaluate at least their entire first page of tweets and some of their followers, but I never get anymore than 10-20 followers in a day, about half of those are blocked. Spammers/Dummy accounts are a real problem on twitter. For some reason, I find people with a vast difference in follower to followed ratio to be more appealing than those whole follow 10k and have 9k reciprocal follows - you know how they attained those followers..via mass follow/unfollow.

I simply look at my twitter stream and block those that are just spamming me like mad (those that are just blasting away many many times a day). With 21,000 followers, no one will look through my followers to judge me. If they are that petty, good for them and I don't want to work with them. Anyone can follow you so why even worry that it is a reflection on you?