The only way it can be updating automatically is if it is a Field, rather than plain text.
It is quite possible that there is a bug with the version of Word you are using, so that it ignores the setting in the "update automatically" box within the shortcut.
You can check it quite easily by highlighting the block of text and checking if you can right click and see Toggle Fields etc. in the drop-down list.
I have Word 2002 and the default is Unchecked from the Menubar - Insert - Date & Time menu, and it works correctly for me. At least it is plain text, not a field.
Other things to investigate might be Auto Complete and Smart Tags, if enabled.
SD