GPO for Secuirty Group to Restrict PST import and Export

Hi ,

I have multiple OU’s every OU contains few users. We are migrated our exchange, Now what be want user from Any OU, Who have been migrated to new exchange can’t Import, Export or create PST. For That i have created a Group policy, Now  i created one security group, Add that group into Group policy’s delegated assign read & apply group policy permission. Later add few users in that group from different-different OU’s , User are still able to import & export the PST. note : same policy is working fine on OU but not on security group.
Please let me know which step i am missing.

Thanks in advance