I've gotten rid of plenty of old questionable smokeless powder simple enough. Either sprinkling in on the lawn or in the bushes, or even pouring it out and lighting it. But what about black powder substitutes?
I have all sorts of subs that were my own or belonged to family members no longer with us. Fairly new, to 30+ years old. Pyrodex, 777, Clean Shot, APP, maybe even some of the newer products. I no longer us it and won't again.
So what do I do with it? Same thing, spread it around? "The solution to pollution is dilution."
Maybe take it out on the water in the spring and shake them out?
Don't say "get some fuse".