Many ways to do it I am sure.
But if you have a PC and not a MAC...try to do a “system restore” and pick a date to restore your computer to..that precedes the 1st time you saw this Cortana bullshit.
Cortana is built into later versions of Windows, which is why I still use Win7. It’s also built into my Xbox and Windows Phone, but are only activated when prompted.
I should switch to Linux, but I’m going to need to start with a fresh computer in order to keep everything I have intact in terms of data.
Watch the video for turning it off as I have (Win 10 Home)
If it does not work PM me and I will give you instructions, basically is is in Settings and then Cortana and turn everything off associated