I think the following should work:
In /etc/profile you'll find the following:
# Notify user of incoming mail. This can be overridden in the user's
# local startup file (~/.bash.login or whatever, depending on the shell)
if [ -x /usr/bin/biff ]; then
Either comment it out, or you could make biff nonexecutable:
chmod -x /usr/sbin/biff
Hope this helps.