Disabling the store will help, but what you want to stop is autodiscover. This is part of Outlook 2007 and 2010 and not something that can be turned off, but not allowed to connect.
What we did was point the Autodiscover URL to a different site (they were going to a hosted solution, but you can use a fake URL) and then when the client prepopulated the fields, if you edit the address, it allowed you to then set the user up manually...
Here was a previous discussion on this that may be of assistance.
http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/Server/SBS_Small_Business_Server/Q_24576087.html
