For starters, they are banned by the bioweapons convention from the 1970s (180+ parties).

Edit: I think back then the rather unpredictable nature and the little added value in deterrence etc. led people to the conclusions that arsenals of those things made little sense (and there was/is public dislike, too). The use was already banned by conventions from the earlie 20th century and the convention then addressed production, development and stockpiles (incl. delivery systems, I think).

I guess all I wanted to point out is, that there can actually be agreement to ban certain technological things pretty comprehensively (outside of some peaceful protective research etc.). Whereas things like SALT are (or were in that case) limiting the number of weapons deployed.