Disable Device Enollment Notification Window
To disable enrollment pop-ups, I moved 2 files...

/System/Library/LaunchAgents/com.apple.ManagedClientAgent.enrollagent.plist
/System/Library/LaunchDaemons/com.apple.ManagedClient.enroll.plist


to /Library/LaunchAgentsDisabled and /Library/LaunchDaemonsDisabled (make sure to create the directories if they don't exist).

Since I am using El Capitan, I am not "allowed" to modify the System folder. I did so by using Target Disk Mode and connecting to a Yosemite machine. You could also do so by disabling System Integrity Protection (instructions at http://osxdaily.com/2015/10/05/disable-rootless-system-integrity-protection-mac-os-x/ )