Prevent Change Packages before Item is saved

I want to prevent that an Item can be associated with a Change Package before the Item is saved, so Change Package -> Create... should be disabled. I first tried to make a mandatory field inaccessible when the condition !(ID>0) AND SICPCount()>0, but unfortunately this is an external information function and can't be used to limit the editability or relevance of a field. Are there any possibilities to do this beside using a trigger script?

Thanks!

Stephan Knopp