See: https://forum.avast.com/index.php?topic=156014.msg1130913#msg1130913
So there's nothing I can do about this mandatory update?
If you read that post in the link it says the last major build of v7, 8 and 9 are continuing to be supported for now, then goes on to say so is build 1367 for v6... or 6.0.1367
You said you were using v6 but didn't say which build. If you were using any other build but the one above, that might be why you got automatically updated to v9. You might try uninstalling AVAST (using their uninstall tools in SafeMode), then download v6.0.1367 from here (
http://www.oldapps.com/avast_antivirus.php) and see if using the proper version 6 will stick.
Alternately you could uninstall 9 then download and install v7.0.1474, which is still being supported and looks very much like v6 so the UI will be familiar.
Whichever you choose, get offline before you install it so you can change the settings -- esp the "don't upgrade the program" setting -- before getting online.