Hi Ed,
I cannot find a way to fixate a body either - but we might as well be both blind. 
I think sooner or later the Shapr3D-Team might provide us with parametric mating and with this a way to fixate bodies is likely to come. But maybe @Istvan can give a little forecast on the roadmap in this regard?
One question: Is this topic here the same as this one?
If so, maybe someone could merge the threads?
Until a possibility to fixate bodies is implemented it will remain best practice to take care while using the move/rotate-feature.
Cheers Matt