Untweeps is a free online tool which helps you to remove inactive Twitter friends, people who have not posted tweets recently. There’s the option of unfollowing users who have not updated 30, 60 or 90 days.
All you need to do is to login (via Twitter Oauth) and then the tool will spit out a page listing all the inactive users.
After which, select which users you want to unfollow and with a click you’ll mass unfollow all the Twitter users selected. While I don’t see it as burden to have inactive users, you might want to clean up your Twitter list and keep only the active users around. Untweeps works for that purpose.