Ping is used to check if a machine is up, Most of the hackers are using ping for finding the hackable machine. The hacking chances can be reduced by blocking ping.

in ubuntu ping can be disable by adding following line to /etc/init.d/networking (network init script)
echo 1 >/proc/sys/net/ipv4/icmp_echo_ignore_all

to re-enable

echo 0 >/proc/sys/net/ipv4/icmp_echo_ignore_all