Deny Access to SSRS reports to Active Directory User in SQL2012

We have ActiveDirectory training accounts used for compliance training for new employees. HR has brought it to our notice that these accounts are able to access SSRS reports. As we are between a new DBA joining I a developer have been given the task to deny them access. With my limited knowledge I got the AD groups that Training accounts have access to. But none of these are visible in the report server database or the report manager URL- site settings . I have tried deleting any users in the site settings except System Admins but that has not made a difference. I have run Deny connect to [Domain\Training1] in the report server but that has not made a difference. In the site settings we have Builtin\Users which I haven’t touched as it is not AD. How can I proceed further ?