@Rich4805
I would continue to escalate this. First, is marijuana illegal in your location?
Take screen shots of the guest messages mentioning drugs
Take screen shots showing Airbnb cancelled the stay
I would quote violations of these two policies:


Flag Guest Profile & Guest Messages
BE SURE YOU HAVE SAVED ALL YOUR SCREEN SHOTS FIRST
I would also consider flagging the guest profile and guest message for violations of Airbnb policies regarding illegal drugs. The guest should be banned. If they are banned, their profile will disappear and so will their review I believe. I think after you report a message, the message disappears. Perhaps hold off flagging until you have exhausted all attempts to have the review remove.

Future Recommendations - Add to Addl House Rules
I would add a statement that illegal drugs and activities are prohibited on property to your addl House Rules. Then when a future potential guest messages you in the same scenario, you could also state the guest intended to break a House Rule (as well as violate Airbnb policies regarding illegal drugs).
If marijuana is legal in your area, then state NO SMOKING OF ANY KIND - including marijuana and vaping.