Disable catchall address for all users without one:

cd /etc/valiases/
for x in *; do sed -i -e "s/\*:\s\w*\w*$/\*: :fail: no such address here/" $x; done

Empty catchall inboxes (mbox):

for i in `ls /home`; do cat /dev/null > /home/$i/mail/inbox; done

SPF Example:

domain.com. IN TXT "v=spf1 a mx ~all"