Private Class??

I’m not sure why I can’t figure this out…
but is it possible to create a Class Object that cannot be instantiated by a user, but ONLY used as a subclass for other classes?

Class genericAuto
   property mfg="nobody"
end Class
Class Mustang as genericAuto
property mfg="Ford Motor Co."
.... overridden or new attributes .....
end Class

Class Impala as genericAuto
property mfg="Its a Chevy"
.... overridden or new attributes .....
end class
let aaa = new genericAuto // this should NOT be allowed
let bbb = new  Impala
let ccc  = new Mustang

Make the Constructor protected and add a Constructor to each of the subclasses.