Hi,
In standard, other than authorization control, there is no option to restrict the usage of purchasing group. You may check whether you can control the usage through authorization.
Else, you may achieve the error message through the BAdI mentioned. I would suggest to maintain a Z table, where you can maintain the combination of plant & purchase group (or any other field) and write a validation in the BAdI to check the entry in PR with the Ztable and issue message as required. Get help from your technical team and proceed.
Regards,
AKPT