This may still work

sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate AllowPreReleaseInstallation -bool false

Should also work: create a restricted software record with "Install OS X*" and uncheck "Restrict exact process name". This will block all OS X installers, beta and current one's.

This is now built into Config Profiles that you can deploy from the JSS, but its tied in with the Software Update payload, so it would mean setting a SUS for your Macs and scoping appropriately. Make sure to uncheck the "Allow installation of OS X beta releases" box as seen below.

If doing this with a Config Profile isn't an option, you'll need to look at creating a specific profile with this setting. This can be done by making a plist with the one setting as @tkimpton posted above and converting it to a mobileconfig profile using mcxToProfile.

You can also upload this config profile and scope it to your users:
Prevent OS X Beta installations

Confirmed it works for EC.

also, it does look like it uses "InstallAssistant" as the unix executable when you run the Installer app,
so you could also block that process name.... (it would also block Yosemite Installs)

So far I've had good luck with just adding Install OS X 10.11 Developer Beta.app to restricted applications. Thanks everyone for the suggestions. I will be trying several of these if I run in to any problems.

Thanks Mike great idea with the config profile :)

For me, just defining the Install OS X 10.11 Developer Beta.app process name worked for me. However, if I renamed the file to let's say, Install OS X 10.11 Developer BetaTest.app then it does not stop the execution or process. There has to be another way to specifically restrict the installation of any software - kinda like applying non-admin rights I guess... That's never going to fly though on our users computers...

I have scoped the config profile as per the Apple kbase article and that works fine to restrict the installer in the Mac App Store. I decided to add Install OS X 10.11 Developer Beta.app to restricted software but that doesn't seem to be working. once I clicked through the installer however, I came across this when I got to selecting the drive. (note the wrong description of the OS)