I have been told on a number of occasions that you can not place a shutoff valve in front of a thermal expansion tank on a hot water heater. However I cannot find that specified in the 2005 code. If you know where that is stated in the code could you please give reference number. Thank you

You aren't going to find this in a code book -- This is an Addendum to the code added by the local authority.

And it makes perfect sense if you think about it from the inspectors point of view.

The reasoning behind the Addendum is to prevent someone, in most cases the homeowner who is ignorant of such things, from circumventing the safety features installed on their system.

You should be able to obtain a copy of addendums made to the code adopted in your area (for a nominal fee, of course) from your local building department.