Gary, try some Spam killer. I suggest Spamihilator. There is a off-topic forum about it and its configuration. You’ll get rid at least of some spam and after avast! processing, you can avoid downloading of the infected messages. So, you can by-pass the warning window. I do this!
Take a look here: http://forum.avast.com/index.php?board=9;action=display;threadid=3413
Spamihilator: http://www.spamihilator.com