"This is Spam" greyed out in Junk E-Mail

I am using NIS 2010 in Win 7 with Office 2007.  I use Outlook for my e-mails. I have just noticed that in Junk E-Mails the "This is Spam" option with the blue cross is greyed out. "This is not Spam" with the green tick is available but I have an item in Junk E-Mail that is unwanted.  Why is the "This is Spam" option greyed out? Surely I should be able to identify unwanted e-mails as spam?  Any help please?