WordPress MU

I looked at some of the WPMU sites listed here. They look great. But, except some active bloggers, most of the blogs either have a single "Hello World" blog or some irrelevent information. They take a lot of system resource.

Is there any plugin or a way to list the blogs not posted/visited in last 30 days. Admin decides which one of them can be deleted or suspended.

Once a blog is deleted, does it completely free database tables and other resources used by the blog?

In the Site Admin area, where the blogs are listed, there's a "Last updated" column. If you click on it (it's blue and underlined) the list will resort and all the blogs that haven't been updated in a while will be at the top of the list.

And next to that, there's nice little buttons that say deactivate, archive, spam and delete.