I guess you would have to override UpdateElements in Model class. It is called just before any changes are commited (at least this is how I see it), and you can ignore or modify the changes that are to be done to the model.
You need to inspect RenderDesign.Actions and discard them if there is a line to be created that is not docked to any element or port. I do have similar mechanism in my implementation and it works very well, but I've done it a long time ago, so I'm not sure if I gave you enough information.
Best regards,
Ireneusz Patalas