Enable or disable SELinux

Edit /etc/selinux/config and look for the line:

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=enforcing

Yes, the comments in the file say it all, set enforcing to enable it, set permissive to log violations and disable to turn it off completely.