IMHO, Federal regulation of commerce with foreign nations must include the power to prohibit, in order to be able to effectively answer prohibitions placed on our own goods by other nations. Among the several state no such power is necessary. They have to authority to lift any prohibition imposed by an offending state.
I think you're right, and that is probably why there are no qualifiers in the Constitutional language. The power must run the full gamut, from very minor sanctions to complete prohibition, in order to effectively counter the array of economic weapons a foreign nation might bring to bear against this country in a protracted economic conflict. Flexible response and a system of graduated deterrents has been an effective tactic in all conflicts. The Founders likely did not want to limit the options of Congress in dealing with economic threats, because they realized the danger those posed to national sovereignty. Many today seem to have lost that perspective, much to our detriment.