You can put the inner shape into a group, that's what I always do. Its points will still be selectable, but certain operations (moving points etc.) will be disabled so you can't accidentally change the original shape. Just select the shape and press "g" to create a new group with one child shape.
Other than that, the only possible way to completely "lock" a mesh is to convert it to a parametric shape. That's not possible in Anim8or, but there is a way around in ASL. BOB_I_Ts recently wrote a script that exports any shape as a parametric mesh plugin. You could export your shape, re-start Anim8or and use the newly created plugin to build a read-only copy of your shape...