The whole mails cannot be deleted (this is IMHO impossible for a generic mail proxy program to do) but you can instruct avast to automatically remove all the infected files from it.
To do this, start avast, switch to the Enhanced User Interface, go to the Tasks folder, right-click 'Resident Protection', and select Change. Then scroll to the Internet Mail provider (in the tree on the left), Virus page. Remove the Interactive selection item, and instead insert the Delete (or Move to Chest) item.
Save the changes and you're all set.
Vlk