I found this forum while researching the problem described here. I don't have the problematic computer with me...but but I have an idea. What about unsubscribing from the spam IMAP folder in Gmail?
Click on the Email account the the Mail Folders view.
Right click on the top level > click on IMAP Folders
Click on Query
Select the spam folder and click unsubscribe
I will post the results once I can try this out in person...
EDIT: it looks like IbnTech and I were thinking along the same lines