If someone you are being forced to collaborate with is being as willful as a child, then you need to be as strict as a schoolmarm. It might not be too much fun to parent a grown adult, but doing so is much more desirable than the alternative -- dealing with the mess that they make. Some people need discipline in order to focus, and you need to be prepared to dole some out. This is good training for a leadership position anyway, so get comfortable having an iron fist!