How do I lock my avatar's shoulders?

I’m trying to make a robotic avatar model. I’m not exactly going for rigid movements, but the default shoulders are a bit much.
In order to make the model compatible with the 3.0 SDK, I had to give it shoulder bones, which it didn’t originally have. Having the arms move in the way they now do was not intended when designing the torso, so the arms are constantly leaving their sockets and clipping through the upper torso.
One time, while messing with a version of the model in the SDK, I managed to shrink the shoulder bone (or perhaps delete it?) and somehow mirror it to the other side. I think it had something to do with the pose option in the bone-binding inspector. I’ve been able to do that twice, but never again after that point. On those models, the arm rotation happens only where the upper arm starts, rather than where the shoulder bones naturally are positioned in the SDK (or, at least, if the shoulder bone is just really small and still bending with the rest of the arm, it’s so small that it’s impossible to tell. If that’s my only option, then I would like to know how to mirror it/where the stats for the bone’s size and positions are)