It would be [i]way[/i] cheaper to just make the crack supply lethal. Of course, then you'd run into the problem of crack addicts quitting crack as they saw fellow users dying off in droves. You can counteract this by "dusting" low-income areas with dust-form Po-210, or possibly ricin/anthrax if that's easier for you to get. Odds are you'll get all of them except the oddball high-income user or two. Of course, this still doesn't address the underlying problem, which is people having the option to do things you don't like. The best way to counteract this would be to construct numerous Pu-239 fission weapons and dope them heavily with Co-59. The extremely high fast-neutron flux would result in a large number of the Co-59 nuclei undergoing neutron capture, thus producing Co-60, one of the highest activity vs. mass isotopes you're likely to run into. Seeding the Earth with these weapons would render it all but inhabitable if done correctly, and the current state of human space travel/terraforming capabilities virtually ensures you'll have achieved your goals.