Wouldn't an embargo against nations that de facto support poaching help kill off the demand for contraband items?

First, I don’t really fully get your question. Embargos on nations that promote or tacitly support poaching would seem to restrict the supply, not the demand.

The problem with an embargo is that they are usually by one (or sometimes a few) countries vs. one country. All you need is one country near the embargoed one for the plan to fall apart.

Like this:
1. Country X is a ruthless dictatorship. They actively support poaching of elephants for ivory.
2. US, Germany, and France places an embargo vs. Country X.
3. Country X can just sneak the stuff to nearby Country Y either through the black market or through conventional means (as Y has not embargoed X).
4. Country Y will then resell the ivory around the world (perhaps including US, Germany, and France.)

Another possibility would be someone going to Country X and smuggling the ivory out.

I agree with @weeveeship. The demand side would have to change.

Totally different situation, but the ‘war on drugs’ does illustrate that as long as demand is there supply will find a way to fulfill the demand no matter the penalties in existence. The goals are: profit for the supplier and ‘satisfaction’ for the consumer. The only way penalties could work is if the risk and penalties are so great in comparison to the potential payoff for the individual (whether they be the supplier or consumer) they would not take the chance of getting caught.

The embargo can take the form of not allowing those nations who have the most consumers of contraband goods to export their wares through out the international community. Especially if they make no attempt or a feeble attampt to curve the trafficing. If the government starts hurting enough behind the bootleg suppliers and their customers they might set up their efforts if the non-contraband users don’t start feeling the pinch their economy will suffer when their factories can’t move any goods globally.

My point was only that the embargo would not stop the black market for such items. “The value of rhino horn made it enormously profitable to poach rhinos and sell them on the black market.”

The problem with an embargo like the one you mentioned, ‘not allowing those nations who have the most consumers of contraband goods to export their wares through out the international community,’ is the entire country would be penalized for the actions of the suppliers who presumably committed an illegal act (presumed because of your word contraband.) I doubt the international community would back such drastic punitive measures for an entire nation.