Make your box invisible to ping

The following firewall rules will drop ICMP requests.

Iptables:

iptables -A OUTPUT -p icmp -d 0/0 -j DROP

OR drop all incomming pings:

echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all

This is sometimes necessary to look invisible to DOS (Denial Of Service) attackers who use ping to watch your machine and launch an attack when it's pressence is detected