How to block an IP using iptables

iptables -A INPUT -s xx.xx.xx.xx -j DROP

specific port:

iptables -A INPUT -p tcp -s xx.xx.xx.xx --dport PORT -j DROP

allow access to an IP?

iptables -A INPUT -s xx.xx.xx.xx -j ACCEPT

allow access to an IP to a specific port using iptables?

iptables -A INPUT -p tcp -s xx.xx.xx.xx --dport PORT -j ACCEPT

where, xx.xx.xx.xx is the remote IP address and PORT is the port number you wish to allow/deny access to.

block a scanner on your server for example “w00tw00t.at.ISC.SANS” using iptables?

iptables -I INPUT -p tcp --dport 80 -m string --algo bm  \
--string 'GET /w00tw00t.at.ISC.SANS.' -j DROP

Source: http://safesrv.net/quick-how-to-denyallow-ip-using-iptables/