/sbin/ipchains -A input -p tcp --source [IP addr] --dport 25 -j DENY
Where [IP addr] is the ip address of the client you want to deny access to use SMTP, on TCP 25. If you want these rules to be persistent on reboot, add them to the bottom of /etc/rc.d/rc.local
[IP addr] can be a subnet in CIDR notation, or a single IP address.
Hope this helped,
Nathan