I know you can disable AI use of your own liveries, but can you do the same for other people's liveries?
As far I as know there is no option for this. You must unsubscribe to be able to achieve this. But there is another question. I'd prepared some own liveries of these coaches we can't spawn in Free Roam and checked to be used by AI trains, but I really never saw any with my livery. I don't know if it is some kind of bug, or should I search these liveries further in game.
Unfortunately not. There needs to be an option within the CC to make the livery active/inactive instead of having to unsubscribe and trying to find it again to subscribe. It's already been put in the suggestions thread so let's hope DTG take note. I have a nice collection of Green 47's for Peak Forest and I hate seeing them turn up on NTP