I'm floating this idea because of recent posts.

Add some way for moderators to "lock" a thread so it can no longer be commented on using the web interface.

This could be as simple as checking if a certain person writes: THIS THREAD IS CLOSED.

Ideally, if a post is submitted via the mailing list it won't be shown after this point.

The reason I suggest that this may work is that if someone is using the mailing list, hopefully, they would respect the closure as they have probably been around longer.