I'm an avid iPhone user but have never had the need or the desire to use Siri.

I suggest people do what I do, load a profile that disables Siri - easily created using the Apple Configurator tool (under "Restrictions" untick "Allow Siri").

N.B. I've never looked closely under Settings on the phone itself, there may well be Siri off option there ? But I just load profiles as I find its easier for hardening.