Junk mail - Stack with preview when 'select all'.

Using Mail for my Gmail account, I often find junk in the spam sub-folder of my Gmail account. The only way to perge it is to select it and put it in the Mail Junk folder, then 'Erase Junk Mail'.

When selecting all, mail in M Lion now shows the messages as a stack and displays the top-most message.

The thing is, with junk - I don't want to open it as it can give a positive response to the sender, allowing them to know that your addy is valid. Which leads to more spam.

Can I turn off this preview? It wasn't happening in 10.7