Disable ping response

echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all
when ping response is disabled, others cannot ping you, but you can ping others.

to enable ping response:
echo 0 > ...

to make it default, add the below line to /etc/sysctl.conf
net.ipv4.conf.icmp_echo_ignore_all = 1