Right now, using Outlook 2007, the users have to open the item before 'Reply' acts as an email reply. If they simply click the email and read it in the reading pain, the reply button in the Outlook toolbar shows 'Post Reply'. This simply posts a message back to the public folder.
Outlook 2007 maintains the Reply\Reply to All\Forward commands even in public folders, but adds the Post Reply to the left of Reply. You can simply customize the standard toolbar when in a public folder and remove the Post Reply button.
I usually just do Alt+R, that gives me Reply. Alt+L is Reply All and Alt+W Forwards it. I rarely use the buttons. Not the best solution but it works like this all over Outlook.