I don't believe there is a way to currently. Even admin factioned NPCs can be shot and killed. The bases themselves can be made indestructible though but not the NPCs.
I don't know if it would work on the traders themselves, since traders are entities and not part of the base. Worth a test though!
If this does not work make the Trader a Desk to stay behind. Or Windows. With our large Colliders you won't hit the trader at all