Oh yes - as pstein observed, we forked into two topics in this thread. I have updated the Help for the Save As topic, but I forgot about the Model Default Diagram topic.
Sigh... there are two scenarios here (but, I hasten to add, they are the same topic!): firstly, there is a model default diagram set that nobody wants, and secondly, there is a model default diagram that some people want but others don't. The second situation would only arise in the Corporate edition, but the solution requires security to be enabled.
Midnight David's solution for either situation is the correct one. If you don't want the model default diagram to open, create a dummy, set it as the default, and delete it. Or delete the original model default diagram, if it is no longer relevant.
However, in the second instance, define either your own diagram or a dummy as the USER default and, if it is a dummy, delete the dummy user default. User default caps Model default, so you personally don't get the model default.
I will put this information in the Help, ready for the next release.