Perhaps once the person has accumulated more than, say, 30 posts, their account won't be deleted? On top of that, if we define "using" as posting or lurking while logged in, then the only accounts that go completely abandoned will be removed.
If an email is sent to their address telling them their account has been deleted, they can remake it if they really desire, and their username will be available since their account no longer exists (unless it's something like "Cave Story" and someone else has taken it by then).
Overall I'm not sure how valuable a function like this would be, as I have no idea how much space one account with no posts actually takes up (or if IPBoard even supports an account deletion feature), but you might want to take it up with Andy. Who knows, it might be advantageous to all of us in the long run.
And get your H key fixed while you're at it.