If they continue smoking, you'll have to put up with it until their lease is over.In the future, make it clear that smoking of any substance is forbidden inside any structure, including garages, storage sheds, etc.

At the time when the chemical industry discovered farmers as a huge market all sorts of herbicides and pesticides (the are banned today) were introduced, stored and disposed of basically into the ground: the farmer did not want to have stuff growing between the barn and the farmhouse anyway, so seemed like a good spot to kick that barrel over.

Keep in mind that when wanting to do this with a mortgage banks first want to see substance and at least one yearly balance statement.

Despite paying a substantial fee, I found that beyond basic instructional videos and repetitive weekly calls, there was little substance to aid my learning.

And the moment someone discovers toxic chemicals in the ground on a property you own the value will plummet b/c no one will want it. 

I know in New Jersey, you cannot bill if you clean it yourself outside of the actual chemicals used (I.E. free labor).  

It was discussed in one of the lender earning call but I still can't get the substance how they able to change the books. 

Tenant shall not allow any sweepings, rubbish, sand, rags, ashes, bones, grease, or other substances to be thrown or deposited therein.

Try these options before busting out the heavy duty chemicals.

Amazing how many investors, who are supposed to be business people, not just normal consumers, fall for sales pitches that lack substance once you dig deeper.