As suggested by Swati, you can add the flow in production rule-set.
Remove all the connectors coming into assignment shape in the flow which you want to remove. But preserve all the connectors coming out from this assignment shape, it will help you in retaining the flow for existing user or work object in this assignment.
Now, add a new assignment(dummy with no section) shape to complete the flow and make the connection coming into the removed assignment shape to it. It will complete the flow for the new user.
Hope it helps you in implementing your requirement. Kindly notify.