Lock the spline in the CF menu... Up by the name and top icons, blue lock. Then, untick the mesh renderer. This will prevent a recalculation of the spline and it'll stay hidden.
I think that does prevent the recalculation, but I am attempting to not have the spline render when I render the course.
This is what happens:
1. Calculate spline
2. Select spline and un-tick mesh renderer
3. Render the course which leaves the mesh invisible, but the physics of the spline are still there - the intended effect is exactly as desired in game.
4. Re-open unity (mesh still invisible)
5. Turn CF on and this automatically puts the tick mark back in the mesh renderer box and the spline becomes visible (even when locked)
I can un-tick and do it all over again, but I will most likely forget to do that at some point and then there will be a problem when someones hits that spot on the course.
Really, I am probably trying to fix a problem with unity's terrain. Maybe this is a terrible solution, but it works if not for the tedious nature of that blasted tick box.
The original problem is when there is no spline and the ball comes off one mesh (a rock for example) onto unity's terrain and the terrain collider doesn't pick up the ball and it goes into free-fall...
Maybe, I should have started with the original problem and not my work-around...
Anyway, thank you for these suggestions and anything else that might be suggested. I am always glad to learn from the community here