Hi Jai Prabhu,
You can delete those classes By navigating menu path, SPRO---->IMG------->Sales and Distribution-------Foreign Trade/Customs---->Legal Control------>Export control class.
And for more information you can refer document
http://help.sap.com/printdocu/core/print46c/en/data/pdf/SDFTCON/SDFTCON.pdf
Regards,
Manjunath B L