There is currently no formal way to remove the BB shell. However, you could set the diameter to zero to make it disappear. I would suggest unchecking the include box for chainstays to remove the chainstays and instead use the seatstay dialog box to represent what most would consider chainstays. The reason being that BikeCAD always assumes the chainstays will be attached to either a yoke or a BB shell, while seat stays can be attached to the seat tube, top tube, down tube or head tube. For a balance bike, I would assume you'd want to attach them to the down tube.