Hi Anil,
Please refer to the below link for the BADi used in Dispute Case Processing.
http://help.sap.com/saphelp_afs64/helpdata/en/58/f01e3f41989310e10000000a114084/content.htm
SCMG_VALIDATE_C - This BADi might give you an option during case change/creation to apply your requirement.
Moreover maintain all the reason code s before hand in config, which I am sure you have.
Cheers!