Is the address using the old POP system?
Generally a simple password change can stop it from happening, though if it is a POP account it can be more difficult to prevent.
When you say 3 virus scans are you referring to scanning your computer 3 times with the same program or using 3 different programs?
You can also try this form https://service.telstra.com.au/customer/general/surveys/Report_misuse_of_service though they may suspend the email account