S2 Member Cleaner
Clients of mine we're having trouble dealing with too many free users on their S2 Member sites. Free users who never log in end up clogging up your database. I did some googling, but came up empty handed. The forums seemed to have a lot of old or incomplete advice, so I ended up building a plugin to manage the overload of free users: S2 Member Cleaner.
This plugin is super lightweight, simple, and free. Just install it, tell it how often you want it to clean out your members, and let it do it's thing.