To quickly block traffic from/to a specific IPv4 address using iptables, you can use the commands below.
Block incoming traffic from a specific IP:
iptables -A INPUT -s 1.2.3.4 -j DROP
Block outgoing traffic (i.e. traffic initiated from the host itself) to a specific IP:
iptables -A OUTPUT -d 1.2.3.4 -j DROP
To block outgoing traffic to a specific port and protocol, you can also do something like this (the example below blocks DNS and HTTP):
/sbin/iptables -A OUTPUT -p tcp --dport 80 -d 1.2.3.4 -j DROP /sbin/iptables -A OUTPUT -p udp --dport 53 -d 1.2.3.4 -j DROP