Disabling all ping requests is easy (you should also add this to your startup scripts):
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all
If you have sysctl, you could edit /etc/sysctl.conf and add this line:
net.ipv4.icmp_echo_ignore_all = 1
And run sysctl -p. This would automagically disable ping responses from your system at boot time.
To enable ping to only your ISP but deny it to everyone else, you'd have to use iptables and know what IP address(es) request ping. This example would allo ping from any address in the 10.0.0.0/255.255.255.0 subnet but deny it from everyone else.
iptables -A INPUT -p icmp -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p icmp -j REJECT