I never want his "assistant" interrupting my work again. I want it OFF my phone permanently. How do I uninstall it?
I don't know how Cortana is disturbing your work, but there is a feature in the system setting, which is supposed to not disturb the user when he doesn't want: Quiet Hours, you could even set some rules as to who could disturb you. For the Quiet Hours: in Settings > Quiet Hours
And as it was mentioned earlier, it is part of the system, you cannot uninstall it, only disable it.
You could also fine tune Cortana (her notebook in fact) to what information "she" can access.
Some mobile device management (MDM) service providers, such as Miradore (which is free), allow you to set a policy to disable Cortana.
While Cortana cannot be removed from the phone, when it is disabled by policy, it will no longer be displayed in the application list.
(You can also use MDM policies to enable device encryption and to disable other features that are potentially privacy-unfriendly, such as Wi-Fi Sense and SQM data reporting.)