configurations could be protected by the password you can set right clicking the 'a' blue ball and choosing the proper item.
that's my way for now, but these way seems only a "workaround" for me

for me, "limited accounts" should not have the right to meddle with avast! (with or without a password) since an antivirus is a critical part of a windows operating system. and one more thing is, limited user can also set a password for avast! what if i've not yet set a password and one of my users set a password which i don't know? that's one of the reason why imho limited account should not have the right to meddle with antivirus settings

Which avast version are you using?
i've installed avast 4.6, and upgraded to 4.7 via iAVS update.
Where did you install avast (path)?
It's on the default location (C:\Program Files\Alwil Software\Avast4) the DATA folder on that folder is fully accessible by everyone.