Tenant complaining of Marijuana smell

Hi everyone, I am getting complaints from a tenant saying the 4 plex (smoke free) smells like weed, and that it is going into their apt. When I originally found out I sent a text out to everyone reminding them that the building is smoke free, and that if I continue to get complaints I will get the police involved. I know which tenant it is (can’t 100% prove it, but I’ve smelt it on him one time when he walked by me into his apt, plus the other tenant said they saw him smoking outside), but he texted back and said he doesn’t smoke and will continue to respect the property. What is happening is he is smoking outside of the property and coming into the property causing the smell to be there. 

The state the property is in marijuana is illegal. What options do I have? It’s a nice 4 plex in a great area and I want to keep it that way. The tenant who is smoking outside of this is very respectful and always pays rent on time!

Thank you in advance!