Hi Milind,
You will have to export the user group files from Ariba as mentioned by Vijay. To do this, please follow the below steps:
-Click on Core Admin->Data export import and then search for user group. You will find the user group related csv files as mentioned in the above response. Open these files in excel and then you will find the different users with different user groups. You will have to edit the file and upload this using the import user group mapping task. Once the import is successful, you can see that now the users for which you had removed the groups from the csv file will no longer have those groups assigned. Please do it in the test realm and if you are happy with the results then use the same file to import in the production environment.
Hope this helps.
Regards,
Nadeem Ahmed