Okay, it doesn't actually disable the logs, it just moves them out of the mail.log file… as described here.

echo "# extract then discard stats from anvil daemon" > /etc/rsyslog.d/maillog-stats.conf
echo ":msg, contains, "statistics: max" /var/log/maillog-stats" >> /etc/rsyslog.d/maillog-stats.conf
echo ":msg, contains, "statistics: max" stop" >> /etc/rsyslog.d/maillog-stats.conf
service rsyslog restart