Blacklist all attempts to login from users outside the US (by IP address)

I'm using WP-Defender (great plugin!!) on my WPMU network and seeing a lot of attempts to login every day from IP's that are outside the US. None of my clients login from outside the US.

I would like to either:
1) Blacklist all countries other than US from attempting to login by IP blocks, or
2) on first attempt to login from international IP, black immediately.

I understand that a wide-range blacklist like I'm wanting comes with some of its own issues -- that's OK.
How would you recommend I proceed?
Thanks,
Larry