Whilst I don't use this function, there are better methods of doing this than the site blocking in the web shield. Given it is the web shield that does the checking and blocking, my guess is that it is likely to be in the WebShield.ini file.
You don't say what OS you are using as the locations differ, you can check these file locations:
C:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\WebShield.ini for XP
C:\ProgramData\AVAST Software\Avast\WebShield.ini in Vista, win7 and later, the ProgramData may be a hidden folder, so you would need to change the windows explorer settings.
EDIT: it is in the WebShield.ini file.