If you wanna clean up a lot of fake or inactive user's registrations (usually made by spammers) by one operation - this tool will help you to do
When your project lives long time, site will get a lot of fake user's registrations (usually made by spammers). This tool will help you to clean this mess up. It can filter, select and delete packs of users.
See more information on http://shra.ru/hobbies/plugins/wordpress-inactive-user-deleter/ - plugin home page.