This feature can be of great use for those sites that are always being spammed by bots when it comes to registering or to remove accounts that haven't been been active in a long time. Only the main admin should have the ability to do it. As for how it should be implemented, it can be used as a check box system in the members list, or it can be added under the Moderate Members area, which would provided advanced search options for the members to delete.

To add to this, in the search, let one way to be by last active

Over all though I think this would be best done via the members list, using a form of User Control Panel. That way it would make use of the members list sort options

I've always told my members never to worry about being inactive. I could see a use for this in the professional genre of forums, but to me "your every day" forum shouldn't need a feature like this.