Which mail statistics are you looking at? (I know of 3 ways to get mail statistics - from server-manager, from the 'qpsmtpd/state' log file, or using the 'spamfilterstats' contrib).
You can *probably* clear out all email-related stats using sv stop qpsmtpd
sv stop qmail
rm -f /var/log/qpsmtpd/*
rm -f /var/log/qmail/*
sv start qmail
sv start qpsmtpd