There may already be a law on the books in some countries.
Regarding the US, these treaties will probably effect more harm to the protections and government as defined by the Constitution than any army. So at what point does that count as "levying war"? Or are we going to pretend that "war" doesn't include economic weapons?
(I suspect this might be easier in other countries, that use a less restrictive definition of treason)
Do you genuinely think it would be reasonable, just and workable to extend the definition of war to drafting a proposed trade agreement that you don't like?
This is more than a agreement to not levy tariffs on each others trade. Since it includes a loss of sovereignty for the US government it should be directly incompatible with the US constitution..
Also, the "nasty people" believe that such laws do not apply to them. This belief is not entirely unfounded. The laws may technically apply to them, but they are somehow never enforced when it really counts.
I don't think you should punish people with ex post facto laws, and in many countries you can't.