Click on the Delegation Tab of the group policy you have created.
If 'Domain Admins' is listed, click on the 'Advanced' button.
Select 'Domain Admins' in the list and then click on deny 'Apply group policy' and click on 'OK'.
If 'Domain Admins' isn't listed, add them and then follow the above process.
You may want to do the same for 'Enterprise Admins' as well.