Mike,
I looked into this and learned that when you are at the stage of "Select Promotion Objects" the system doesn't really care (or know) what the purpose of the collection is (Add to Workspace, Promotion Request, etc). Therefore, it's difficult to stop the collection because there's no way to know if the collection is for a Promotion Request.
However, if the objective is to stop objects in a specified context from being promoted there is a way to do that.
A listener can be written to listen for a POST_STORE event.
The code in the listener would check if the object being stored is a Promotion Notice.
If it is a Promotion Notice check the objects being promoted.
If any of them are in the specified context remove them from the Promotion Request.
That said, if you want to prevent users from adding objects from the specified context to the Promotion Request after it is created it might be more bomb-proof to check for the creation of a PromotionTarget, which is the link that links the Promotion Request to the object being promoted. If the object pointed to by the PromotionTarget is in the specified context, delete the PromotionTarget.
Not sure if this would be good enough but if the goal is a bomb-proof way of preventing objects in a context from being promoted this would do it.
If this sounds interesting and you need more help feel free to drop me a message.
David
