iptables used to setup, maintain and inspect the tables of IP packet filter rules through Linux Kernel. You can modify or manage the rules through the command from console or SSH.
To list the IPs that is being blocked, you may run the command as below.
iptables -L -n
You will get the list of the IP that is being listed from the Iptables Kernel rules as command above.
To block an IP using iptables,
iptables -D INPUT -s xxx.xxx.xxx.xxx -j DROP
where xxx.xxx.xxx.xxx is the actual IP