Can we set the default value of a field to none?

In Menu->Display->Custom fields you can set the Default value for fields. Is it possible to set this to "none" or something similar which then requires that the end user takes some action. In other words I would like to not have a default value yet still have it as a required field.

Most of our sales are in Australia and there is no logical default value for state when Australia is selected as the Country. My problem is many of the sales are being processed with the default state selected when this is not actually the case at all, thus causing problems with address details.