Trying to quit persistent mode, for me, gives me the following message:
You have selected to turn persistent mode off. avast! will NOT be launched automatically at log in unless you turn persistent mode back on.
To do this, the application has to be quit (you will be able to launch -- or quit -- it manually any time).
It then quits the application but when I restart (maybe I shouldn't?) to check the persistence mode setting, it comes up as turned on. I'm running 10.6.2 ...