Hello Amol,
It's not possible to delete specifications for which reports with status Completed or Historical are assigned. If you want to delete, firstly, reject the report in the Report Management.
You can also check the following documentation in order to get further details about this process:
http://help.sap.com/erp2005_ehp_04/helpdata/EN/ec/32b23a2410ab26e10000000a11402f/frameset.htm
Best Regards,
Camila Zilles