Directly through iptables or if you're using Bastille firewall by adding this to etc/Bastille/firewall.d/post-rule-setup.sh...
/sbin/iptables -I INPUT -p tcp -s IPADDRESS --dport PORTNUMBER -j DROP
Then /etc/init.d/bastille-firewall restart
The options for -j are ACCEPT, DROP, REJECT and RETURN (jump to another chain in iptables).
Be carefull you can block more than you want to. For example I ACCEPT mail on port 25 from howtoforge.com but DROP all other ip addresses in their CIDR (184.108.40.206/8) that attempt to access port 25.