Prevent automatic OS addition

in my Foreman setup I sometimes define a custom OS provisioning
configuration, e.g. a "myRHEL 6.3" OS which is actually a RedHat 6.3 with
the custom settings I need.

But then, after my new VM has been provisioned, it happens that Foreman
somehow (Puppet facts, I suppose) becomes aware of the base OS being used,
and so the OS setting of the VM is automatically changed to use a "RedHat
6.3" OS, which I never configured in Foreman; so my custom settings for the
OS are lost.

Is there any way to prevent this from happening, so that OS names are not
limited to what Foreman can discover from fatcs?

Thank you.