Kills Mail.app on Mac OS X for a minute, every 5 seconds. Schedule this as a cron job to run every minute during your focus hours. You'll open up Mail, and it will quickly close it for you.

killmail.sh

#!/bin/bash
# how many seconds to sleep for between quits
SLEEPTIME=5

for((i=0;i<60;i+=SLEEPTIME))
do
  osascript -e 'tell application "Mail" to quit' > /dev/null 2>&1
  sleep $SLEEPTIME
done