You have to create a filtering rule which tells ClamAV what to do with the viruses it detects: To do that:

1.) Navigate into the Webadmin interface to Security & Filtering -> Incoming Message Rules

2.) Click the 'Add Message Rule' button

3.) Write a suggestive name for the rule

4.) In the Conditions section select "Custom"

5.) Write in the first textbox "X-AxigenVirus-Level" without quotes

6.) Select "Is" from the combo box

7.) Write the value "5" without quotes in the last textbox

8.) Select Delete or Discard in the Actions section

9.) Save the rule