Disable or block outgoing email
Solution 1:
I guess something like that would do:
iptables -A OUTPUT -p tcp --dport 25 -j REJECT
dport is the destination port 25, for SMTP. Packets will simply be dropped. To apply the rules you need to run:
/sbin/service iptables save
Here's the CentOS iptables documentation.
Can't test it, but should work.