I should have probably put an example, I'm sorry. Bad clarification on my part.
Let's assume that two subassemblies, A and B, are placed and have an Up/Down hinge connecting them. LDD likes to drag BOTH A and B the way you rotate it, one more drastically than the other. My thought was to remove the minor, unwanted rotation by making the half I don't want moved 'fixed in space'.
Ctrl+Z, while it undoes the unwanted rotation, isn't useful because it doesn't help me achieve my goal. However, if I were to 'ground' a single piece in subassembly A, then subassembly A wouldn't move by rotating means. You could still drag it about, however. It would also be useful to make the grounded part visible and easy to find; just in case if you want subassembly A moveable later...