If you run "Get-AppxPackage -Name Microsoft.Windows.Cortana" in Windows Powershell (Run Powershell as Administrator) you will find the true path to Cortana's exes and DLL files.
You can then use the Everyone DENY permission to block the executables from running. However this should be a last resort and may cause future update/upgrades to fail. (unless you revert the permission before applying said updates).
For instance in my configuration Get-AppxPackage returns
InstallLocation : C:\Windows\SystemApps\Microsoft.Windows.Cortana_cw5n1h2txyewy
In my experience setting Deny permissions on the executables is usually enough, however by default TrustedInstaller owns them and you will have to take ownership before you can edit the ACL (Access Control List) to add an Everyone Deny permission to prevent them from running. This can cause other unintended consequences such as random errors logged in your Event Log as well.
I really don't want to give exact instructions in the thread as I don't want people messing their systems up, if this wasn't enough information for you to figure it out PM me and I'll assist. Myself, Windows Forums or Stack-Tech is not responsible for any damage caused however
You might also be interested in
Spybot's Windows Anti-Beacon which can further block communication to Microsoft. Again, use at your own risk.