How to restrict workitem delivery to a specific stream?

There is nothing built into RTC that does this. The work item also does not have any information that would help a system to do such a restriction. You would have to create your own advisor similar to https://rsjazz.wordpress.com/2012/11/01/restrict-delivery-of-changesets-to-workitem-types-advisordelivery-of-changesets-associated-to-wrong-work-item-types-advisor/ that somehow determines if the change associated to the work item is supposed to go to the stream or non and prevents that if not.