On which side of the debate are you?

Rather than make the debate centre upon morality or international law, the affirmative could easily interpret the "ought" in a different way. For example, a good case can be made that economic sanctions are ineffective or even counter-productive, with references to modern history. This avoids the necessity of dabbling in difficult moral questions.

Another but moral line of thinking might be that economic sanctions generally hurt not the government itself, but its suffering citizens. Even if this results in an internal regime change (it can also have the opposite effect in rallying the people around the flag), the populace still has a grudge that seldom goes away for several generations.