Oh I think you may be cursing the Microsoft and Bill Gates for preparing such a program like Outlook 2007 when you met with an unexpected problem like this. But the solution for this problem is as simple as Outlook itself. I too had the same problem and I was also cursing. There is no need to log on to www.microsoft.com/support for solving this. Either you need not to log on to www.yahoo.com or www.google.com. If you log on to these sites for solution you will be led astray. Therefore I come to the solution.
The remedy for this is to just close the application. As soon as you close the application Outlook you will be automatically signed out. But you have to do one more thing in order to ensure that nobody send email from your account. Go Tools and take account settings then remove the tick from remember your password option. Then you can believe that nobody can access your account without entering the password. Once closed again if you want to send email you will have to enter the password. If you don't enter the password you will not even find the send button in the Outlook.
Then enjoy the Outlook and make merry.