One tool to experiment with is TrueTwit.com. This has both a free and a for-pay aspect to it. You can test out the for-pay side for free for thirty days to see if it's helpful to you.
First off, many Twitter users use TrueTwit so if you try to follow them you'll get a "you must click here to verify you're real" message. By joining TrueTwit yourself you no longer get those messages. You're a confirmed user. So that saves you time there. The basic account will send those messages out to people who try to follow you. Depending on your point of view, that can be good or bad. Yes, it'll tend to fend off people who are spammers - but it might also discourage people who are just normal Twitter users who want to follow you. The only way to disable those auto-messages out to anyone who follows you is to sign up for their premium account.
The premium account gives you access to all sorts of useful tools. For example, you can scan through your followers and see exactly which ones are flagged for being adult content (porn), ad promoters, robotic behavior, or adult language.
You can then either block or wholly unfollow them right from that screen. So it's easy to scan the list and remove those spammers. You might say to yourself, well, but if I drop the spammy people, my follower count will drop. And then I won't be able to follow as many people myself. In the short term that's true. But in the long term if you keep spammers on your account you are helping support the spam system and also diluting the value of your own audience. So it's better to slowly but surely drop out those spammers and keep your audience clean.
It's also important that you, yourself, don't make posts of this sort (unless you are in fact an erotic author) lest you end up on other readers' lists and get dropped by them.
The ratio here is interesting. A high ratio (a high number) indicates a person is following a ton of people but hardly anybody is following them back. That's generally a sure sign of a robotic account. It could also be a brand new account, too, but in any case worth taking note of.
Followers - are you Following Them Back?
You can look at the people who are following you and see if they were sent the verify DM (if you're using that option). You can see which of them you've chosen to follow back, to help coordinate following back people who deserve it. You can note in this screenshot that I chose not to follow back the two "egg" accounts which seemed to be spammers.
People You're Following - Are They Following You Back?
They claim to have a report of who is not following you back but I've never gotten it to successfully work. So while this data might be useful, it's just not filling in for me.
The premium service is $20/year which isn't too much - and it lets you get a better handle on your followers than regular Twitter or TweetDeck allows. The interface is a bit clunky looking. With the system not fully working for me, this isn't something that I'd sign up for long term right now. And I wouldn't use their free service because I don't want to have every new follower get hit by a "wall" that they had to take action on - I wouldn't want to lose those potential new followers. I'd rather just manually deal with any porn / spammers who ended up following me.
Even if you choose not to use TrueTwit, it's important to understand how it works because hundreds of thousands of other Twitter accounts use this. If your tweets are all primarily ads, you're going to be flagged as a spammer. It's important to only have a few ads interspersed with your other "helpful" posts.
Building Traffic for your Twitter Account
Followers and Twitter's Value
What is a Twitter Hashtag?
Maintaining Multiple Twitter Accounts
Choosing a Twitter Icon
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What is a Twitter Hashtag?
Twitter for Authors
Twitter Promotion Tips
Twitter Next Step
Twitter 2000 Follower Limit
TweetDeck - Free App for Twitter
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Tweepi - Manage your Followers
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FollowerFilter - Great Free Follower Management Tools
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