Check your email settings, I thought Outlook (if you have it) has a function where all items in the deleted items box are deleted upon exiting. You could try and set up a rule where specific mail (if you have a particular spammer/unwanted sender) or spam are automatically moved into your deleted items box then automatically deleted on exiting as above.
http://office.microso...rule-HP005242897.aspx
Just be careful it's not too wide that it could pick up mail you need to see.
There are all knds of rules you can create.