Pretty straightforward title, I believe there should be a small button next to where you subscribe to a livery that allows you to toggle if you want AI trains to use it or not, just like you can with your own liveries.
I very much support this suggestion - now that Free Roam's out I want to download interesting liveries, but I don't want to do so if they'll show up on AI trains.
Yeah I head already create a topic about this point direct after EA release... It will be very welcome when you can switch between "active in AI" or "player only"