Does the buttom does not work?
Maybe you can send an email to Vlk and ask him how to disable it into the Windows Registry 
Actually there is no such an option to dislable a password once set up. Set/Change password available when right-clicked on the avast systray icon is the option - that is only Set or Change no Disable.
I know it is kind of trial question to ask, but I will try the Vlk-channel;)