We are using a legitimate copy of CAD software (Autocad Architecture 2008).  However when we open a previous version of dwgs (ADT 2.0) and older ones, we get the message "PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT". How to get rid of such stamp?
 
We have actually succesfully used two methods:
1. by DXF and re-import again in a new drawing.
2. by external referencing the file and bind (insert) into a new drawing.
 
However, if it involves a complicated xref's (embeded), it would be complicated altogether.
 
Autodesk rep says the original drawing was produced by an Educational Copy of Autocad.  Assuming it is (perhaps by other consultants or others outside our office which we do not have knowledhge at all) and use a portion of those documents into our legitimate drawings, is there anyway to fix it? It would be seemingly unfair to penalised legit users of the folly of others.

Your DWGs or blocks inserted in those DWGs were indeed produced in an EDU version of AutoCAD. The commercial versions of AutoCAD always warn before inserting or otherwise "infecting" from the EDU-stamped DWG files.
Your Autodesk dealer can assist you in batch-removing the EDU stamp. Using EDU versions of AutoCAD for commercial projects is not complaint with the license agreement.

Indeed. Thank you so much.
 
However, the files we are actually mentioning was done during ADT2.0. In those times, we have not receive any warnings whatsoever.  When we open the same drawings now in Autocad Architecture 2008, the problem crops out.
 
As much as we do not want to entertain such non-compliant usage, there is no way we would be able to know those things at the time of ADT 2.0.
 
Even batch editing to eliminate the stamp in a heavily xref environment would really be a disaster.
 
Additional info would be much appreciated.