The NOT NULL Constraint clause is presumably appearing because the "Not Null Constraint Name" property is set on the Column. (It's shown on the "Default and Constraint" panel in the Properties dialog for the Column.)
To stop these being generated, you can go to the Data Modeler/DDL page of the Preferences (from the Tools menu) and set the option "Generate Short Form of NOT NULL Constraint".
Note that there now is a forum specifically for Data Modeler: SQL Developer Data Modeler
thank you very much. Now I know where to find the NNC property ;-)
In the table property dialog page that shows all the column names, double click on the column name and that will open the column properties. The not null constraint name is on one of the tabs there.