Hi Subhash,
Yes, you can delete the IDOCs in QA which are copied from PRD.
Follow the below links on steps to delete IDOCs
http://wiki.scn.sap.com/wiki/display/ABAP/Reprocess+or+Delete+an+Idoc
Considering the DB size mentioned by you I believe this deletion might also run for quite a while.
If you are aware of the IDOC tables on which the conversion is not required, you can exclude them from the BDLS conversion before starting BDLS the below steps:
Select multiple selection button 
in the below screen
List the tables to be excluded in conversion
This would save the time on deletion IDOCS.
If you are planning on other alternate ways please do share with us.
Regards,
Pradeep