The proper way to dispose of reject solid propellant is to burn it. How perfect! So we save the rejects of the year for a annual burn, such as a Q-grain with lumpy oxidizer and my cracked titanium sponge grain that destroyed the upper half of my friend's Nest Rocket. Photos: https://www.facebook.com/jurvetson/posts/10153546042440611
We end with a rare 54mm Aerotech single use J125 moonburner (with an impressive 10 second burn).