probably the default (when the key is missing) is enabled. Then, change the value in the disabled key to true, and restart the computer.
This didn't work too, but here is my little workaround to quit avast in this special issue (for people, who must quit avast for some reasons):
1. quit avast by right-click on the dock-icon
result: avast shuts down and restarts immediately
2. quit avast a second time
result: avast shuts down and restarts about 3 seconds later
3. within these 3 seconds, trash avast.app (finder/applications)
result: avast is trashed, but process is still persistent
4. go to apple menu and force quit the avast.app process
result: process is killed
5. restart Mac
result: no avast process
Now men can do stuff where all applications has to be quit (me: install windows by using bootcamp assistant)
After this, replace avast from trash or reinstall avast (be sure, that u have saved the license.dat from your avast license-mail)
Nevertheless, thank u for your time to help me!