Unlike majority of the default Windows apps, Cortana can not be disabled through Powershell. Microsoft wants people to use Cortana.
If you want to completely turn off Cortana, then you will need to resort to a registry fix (Note - It is recommended that you create a backup of your registry before modifying so that if you commit some mistake, you will be able to restore - https://support.microsoft.com/en-us/help/322756...)
Windows Key+R > Type regedit and Enter
Go to the key - HKEY_LOCAL_MACHINE > SOFTWARE > Policies > Microsoft > Windows
See if you have a directory named - Windows Search
If not, then right click on Windows folder > New Key > Give the name Windows Search
Click Windows Search > In the right-hand side pane, right-click in the empty area and choose New > DWORD (32-bit) Value. Type AllowCortana > Enter
Double-click AllowCortana, and type in 0 under Value Data.
Reboot your computer