A step beyond this would be to set it to kill your connection every minute during the time that you wanted to focus. So, you could set your cron job to run every minute while you were in need of focus. So, you could write a simple shell script to alter your crontab to add in the "kill my connection" command. No time to write up a version right now, but it would go something like this:

if [ -e /etc/crontab.bak ]
then
mv /etc/crontab.bak /etc/crontab
else
cp (or mv) /etc/crontab /etc/crontab.bak
{necessary mods to crontab to kill net connection}
fi