Here is a free Twitter software to unfollow automatically unfollow twitter users that don't follow you:

Free Twitter Marketing Software - TweetAttacks Free Edition

The software does not use the Twitter api. I've tested the software and it works get!

There is also a paid version TweetAttacks Pro that has a Reply Generator Module:

Generate replies to thousand of users looking for your product and services Tweet Spinning
Generate thousand retweets

The software has no monthly fees.