I understand that you've had some bad experiences with people who also happen to smoke marijuana, but you are painting with much too broad a brush.
You must ask yourself: how accurate are you in your assessment of who smokes marijuana and who doesn't? For some of your tenants, I'm sure it's obvious that they partake. But, I'd wager many of your 'good' tenants do as well, and you have no idea. It's really not hard to hide marijuana use from your landlord. And from what it sounds like, the tenants who don't make any effort to be discrete with their marijuana use are also the tenants who don't care if their car is blocking another car. Perhaps a DGAF attitude would be a better thing for you to screen for than marijuana use.
You must ask yourself: how accurate are you in your assessment of who smokes marijuana and who doesn't? For some of your tenants, I'm sure it's obvious that they partake. But, I'd wager many of your 'good' tenants do as well, and you have no idea. It's really not hard to hide marijuana use from your landlord. And from what it sounds like, the tenants who don't make any effort to be discrete with their marijuana use are also the tenants who don't care if their car is blocking another car. Perhaps a DGAF attitude would be a better thing for you to screen for than marijuana use.