Add the following line to your init script for the network (the name depends on the distribution you use):
echo 1 >/proc/sys/net/ipv4/icmp_echo_ignore_all
This disables ping responses.
To reenable, use the following command:
echo 0 >/proc/sys/net/ipv4/icmp_echo_ignore_all
To make this permanent set the following into /etc/sysctl.conf (if you have such a file)
net.ipv4.conf.icmp_echo_ignore_all = 1
