You can use DROP or REJECT depending on what you want the blocked host to know.

iptables -I INPUT -s 66.184.141.132 -j DROP

Delete the rules:

iptables -D INPUT -s 66.184.141.132 -j DROP

Show the rules:

iptables -L INPUT

One-liner to block evil hosts grep'd from Apache error logs:

for ip in `grep Itau.com.br /var/log/httpd/error_log|egrep -o "client [0-9]+\.[0-9]+\.[0-9]+\.[0-9]+"|sort |uniq|cut -f 2 -d " "`; do iptables -I INPUT -s $ip -j DROP; done