WordPress gives you the option to allow people to sign up for an account on your site. Unless you run a well-established community, you can probably keep that featured turned off. Once you allow people to register on your site, you are going to get trolls, spammers, and all kinds of dubious characters join your community. We have already covered tips and tricks for dealing with rogue users. Here are 3 plugins you can use to handle inactive users:
No Posts User Delete: deletes all users that have no posts. Just be careful how you use this as you do not want to go overboard.
Automatic Signout for Inactivity: this is a nice little plugin to use to protect your community. It automatically signs out inactive users after a specified time period of inactivity.
Hide Inactive Sites: a plugin designed for those of you who host multiple blogs in your network. It changes visibility of a blog after it has seen no activity after a specified amount of time. It hides inactive blogs in your network for you.
You should still actively monitor your users and sites to handle rogue and less active users. The above plugins lend you a hand with the process.