Hi,
I have recently noticed this also...
I uninstalled it in Programs and Features, and again later in APPS...
It reinstalled in as little as 10-15 minutes..
It does not seem to appear in the Windows Update Show\Hide Tool either...
I can only think it must be hidden in another update...
What worked best for me was to go into Taskmanager, right click on the Windows Update Assistant, and select "Open File Location"...
I then renamed the file and then selected "End Task"... This was successful in shutting it down without it trying to reinstall...
I later found this suggestion concerning Disabling it in Task Scheduler...
From what I have found is that it is best to not Uninstall it, just disable it... Uninstalling simply triggers it to reinstall...