I talked with my wife, the future BCBA, and it appears that my intellectual reach has exceeded my grasp. First, I seem to have confused positive reinforcement v. punishment and positive and negative instruction. It is the case that negative instruction ("Don't throw your toy car") is less effective than positive instruction ("We only throw balls").

Second, there are some interventions, reinforcing and punishing, that could teach in one trial (consider heroin injections as reinforcement and flamethrowers as punishment). Edit: my wife says this point is about salience.

Third, best practices among behavior analysts are to use reinforcement prior to using punishment. My wife says that this is for ethical reasons - her reference book didn't talk about the relative effectiveness of reinforcement and punishment.