You can set a policy to disable the menu command. The command bar ID should
be 5514. The policy templates can be found in the Office Resource Kit http://www.microsoft.com/office/ork. You will want to download and install
the Office Core Tools to get the policy templates.
I don't think the policy is 100% bullet proof because it only turns off the
menu item(s). If the user already had the preview pane enabled it will still
be enabled when the policy is turned on.
Please let me know if you need assistance in getting this policy enabled.