I realize this is old but wanted to post the answer here for anyone else that might need the capability. It would have saved me some time when I was searching for the answer earlier this week. I finally figured out half the equation and Google Support offered the other piece.
You can achieve this via the Content Manager. Setup a Rule to block Sender based on Regex with this criteria:
The above regex is matches .in, cn, and dk domains. Add .ru or any domain that you want.