CortanaKiller

Granted, when Windows 10 was first released a few years ago, you could simply turn off Cortana from the digital assistant settings. However, with the Windows 10 Anniversary Update, this option was removed and left users with an unwanted service running in the background. Since Cortana is gathering information about what you are doing, it goes without saying that you want the service off permanently.
CortanaKiller is a tiny tool designed to assist you in this irritating matter. The program comes as an alternative to a more complicated solution for this problem, which typically involved editing registries.

All you need to do is run the executable

The application does not come with an interface, but rather it acts more as a service that you can find in the Task Manager. Therefore, once you turn it off, the only way to toggle it back on again is to uninstall the program. Alternatively, you can open Task Manager and kill the service from there.
It is important to note that in some configurations, the application tends to eat up a lot of the computer's resources. In a way this is relatively normal considering that Cortana tends to randomly open.

A possible solution to ensure Cortana is not running in the background

Over the years, Microsoft has worked hard to improve the service and we must admit that they succeeded for the most part. Then again, there are numerous users out there who simply do not want the service maybe because it does not feel like a proper AI assistant or perhaps due to more or less legitimate concerns of data collection. Although the latter was not proven, it still managed to enrage a lot of users.
Regardless of your own reasons why you do not want the Microsoft AI running in the background, CortanaKiller is a tool that might be worth a try.