Close all Form in dll.

There is no direct way to do that.
However, you could add it yourself - it's not that difficult.
Create a static list within the DLL (it's probably worth adding a static class "DLLControl" or similar for this) of Form objects.
Each time one of the forms gets a Load event, add it to the static list, and add a handler to the FormClosing event.
In the FormClosing handler, check if the form is in the static list, and if so remove it.

Now, add a static method to the DLL:

public static CloseAllForms()
   {
   List<Form> openForms = staticListOfForms;
   staticListOfForms = new List<Form>();
   foreach (form f in openForms)
      f.Close();
   }