How about a bioweapon with a (but utterly ridiculous and outlandish) weakness built in through creative application of biotechnology? It would make prevention fairly easy for you and still keep it semi-impossible for the enemy.
How about a bioweapon that destroys all harmful bacteria and viruses in the target area (including cultures and such in labs, vaccines, &c.), and keeps it "clean" for at least several centuries. If its deployment is undetectable, that has the potential to be a
really nasty sneaky way to completely destroy another state, at least if you're willing to play the long game.
The weakness is that a slight, known, and very carefully guarded alteration in the genetic structure of any of those viruses or microorganisms renders them immune to the effects of the weapon, so that if the weapons are reverse-engineered or captured and deployed against you, they can be countered easily and without damage.
In the medium term (2-4 generations) it will slowly push the target society towards isolationism and xenophobia, given the inevitable association of foreigners and immigrants with outbreaks of disease. Diplomatic and trade missions will be reduced in number and size, international tourism and employment from that country will die out, and those that do leave will eventually have to return, as well as taking precautions while abroad which serve to distance them from and dehumanize them in the eyes of their counterparts.
This will continue to escalate over generations until the target is effectively an island, with little or no face-to-face contact with the rest of the world.
When the weapon's effects end, all it will take will be a handful of infiltrators and germ bombs to largely destroy the population. The only alternative, if the effects were noticed within a generation or two, would be total immigration out of the affected area, which (given the lack of unowned land in the world) would destroy their society and government as well.
Of course that's purely theoretical and depends on a number of unspecified advancements in the life sciences, but it's a rather elegant thought.