Hi Nav,
Please follow below mentioned steps to delete already used reason code :-
1) Mark the Catalog "Inactive" to which the Code group->Reason code is assigned.
SPRO->CRM->Transaction->Settings for activities->Define activity reason->Define Catalogs
2) Select the Reason code and delete (You may have to log off and login back again after step-1)
SPRO->CRM->Transaction->Settings for activities->Define activity reason->Define Code Groups and Codes for Catalogs.
3) Go back to the calatog of step-1 and remove "Inactive " Flag & save
Thanks
Prateek