To prohibit users from having access to Siri even from the lock screen is to enable the passcode and then turn off the Siri option. This can be found under settings --> general --> password lock --> Siri --> off. This changes the setting to "do not allow access to Siri when locked with a passcode". Unless I'm not understanding what you are trying to do and indicate is at risk. Maybe this feature is not available for enterprise permission yet. Otherwise maybe this is the setting you need.
From My original post
I have posted this on other forums - but get a barrage of -
"But you can turn it off responses"
Hoping the average IQ here is a little higher and can understand the issue
......There is the option in the settings to disable siri at the lock screen, however as an enterprise we are not able to leave that option available to the end user as it compromises our security policies. What we really need is to be able to disable and "Grey Out" that option - just like we can do with the the passcode setting......
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