Windows 10 Upgrade Assistant is useful to Upgrade install latest windows 10 ( ex : Upgrade to windows 10 Fall Creators update ) smoothly without any errors.
You Are looking for Disable Windows Update Assistant because you dont want the Latest Windows updates or don’t Want to upgrade latest windows 10 version right ?
Then Simply disable The Windows update servcie. To do this pres win + r, Type servcies.msc and hit enter key. Scroll down look for windows update service double click on it change the startup type to disable and stop the service if its running.
This will never start the Windows update service means windows cant check for latest updates and you didn't get the Upgrade assistant popup.More details: How Do I Disable Windows 10 Update Assistant?