I'm using SharePoint 2013
I would to grant acces to a subsite by a specific Active Directory group. And i want to deny all other groups.
So the group that's allowed is able to acces the subsite and other groups don't.
Assuming your parent site have a group A which shouldn't have permission to your subsite, the first thing you need to do is stop inheriting permissions on your subsite. Site Actions > Site Permissions:
Then select Stop inheriting permissions:
And you'll get a warning you need to accept:
When these steps are done, you can safely remove the group that should not have permission to your subsite.
SharePoint's security model is deny all, allow select. Therefore, you can manage the permissions on the site and specify who you want to have access, and if someone is not listed, they won't have access. There is no specific deny option.
If you need assistance in actually walking through how to add a group. Reply and we'll get you the steps.
HTH