Well, I think we should be able to change objects or tweak them slightly, just not remove them completely. For example, let's say someone adds a ball in the toy box, but they don't feel too confident texturing it, so they slap a preset on and pass it to the next person. Let's say it was a wood material. I would say it's ok for the next person to replace it with a similar wood material unless the original person specifically asked them not to alter the object. But no, I don;t think drastic alerations or removal should be allowed. My only advice is to complete your turn to your highest standard so that the next person won't need to change it much.