help! Ssh can not be connected to SELinux

I'm assuming you used CentOS from the marketplace and thus cannot mount the volume as a secondary drive to another instance to undo your change, nor do you have a snapshot. (Otherwise, do one of those).

It appears to can have AWS remove the marketplace code from a snapshot (https://forums.aws.amazon.com/message.jspa?messageID=541676). I would recommend doing it through a support ticket. Once complete, you can create a new volume that you can mount on a working system, disable SELinux, and reattach to the original instance.