In general, don’t worry about trying to control who follows you. But it’s a good idea to prune out the followers who have stopped using Twitter.

A great free tool for finding people who haven’t tweeted for a while is unTweeps. After you authorize the app, it allows you to create a list of your followers based on how long since their last tweet. You can use the free account three times each month.
Start with people who haven’t tweeted for 6 months (or even 9 months) and review the list. You can mark individual accounts to unfollow.

Tip: If you have a large number of people who are no longer tweeting, don’t unfollow them all at the same time. This action can signal Twitter to suspend your account for aggressive and spammer-like behavior.