What prevents a machine from responding to pings?
I dont' think that it's related to ping problem, but if you want to put selinux temporary off, you have this option:
setenforce 0
it put selinux from enforcing to permissive mode, to check its condition run
sestatus
to diable selinux permanently you can use system-config-securitylevel
or edit with nano
or vi
/etc/selinux/config
and change the parameter from SELINUX=enforcing
to SELINUX=disabled
.
for me there is a rule in Amazon EC2 that prevent to allow the ping traffic between your machines...