I needed to performance maintenance+tune up on my pc and wasn't able to close Avast; that's how I found this article.
Result:
The only other software that required a separate uninstaller was Norton's bloatware, that would often leave files/folders/registry entry's behind. Now Avast is behaving similar to malware, to the point where it has taken control of the computer, and doesn't allow a basic function such as exiting the program.
This leaves the user a single & clear option of uninstalling the program. I recommend using Revo uninstaller to completely remove all remnants of it.
Further, there are other local/cloud based solutions, which offer greater performance with smaller footprint.
Panda seems to stand out, and you can use 3rd party software to control it's start up, or disable it.