As far as I know, there is no way to disable MT ghosts. If you want to show a player the way, have him drive through a block that has a trigger for the ghost that isn't part of the normal driving line. Keep in mind that the player doesn't have to be following the example ghost, he just has to see it. A popular method is to put a false route somewhere off the start, and have that trigger the GPS ghost. The camera would then change to this ghost.
I don't have a lot of experience with trails, but I think something similar applies. Perhaps they're only triggered when you drive across a trigger for the trails ...
Hope this helps