QUESTION
 How do I delete a class/part which still has instances ?

ANSWER
 To remove instances of a class in VisualAge, execute:
 classname allInstances do: [:anInstance | anInstance become: nil].

 Although this method of removing instances does work, and all
 of us have resorted to it at some time or another, we should
 warn you that it is dangerous.  You will usually get away with
 this, but it can also cause walkbacks and even system crashes
 or meltdowns.  Use it at your own risk.