With the free version (smiple user interface) there is only two choices. If you under “Internet Mail”, “Customize”, “Advanced”
select “Silent mode” and “with general answer No” then all infected attachements will be moved to the chest (but the message will not be deleted though). To bad there is no “Delete attachment” (you need to empty the Chest from time to time 
BTW: To the avast! team. The helt text is WRONG on “with general answer No”. It’s says the chossen action will be “No” or “Delete”, but it will be “Move to chest”. Maybe you should change this.