Your basic options:
1. Talk to your neighbor and try to work something out.
2. Move.
3. Build a fence (whether it be new or one that's higher and/or better than any existing fence).
4. Report the alleged crime to the police (keeping in mind that if it's medical use, it may be legal), but I agree with "PayrolGuy" that the police aren't going to make this much of a priority.
5. File a nuisance lawsuit.
6. Suck it up and deal.
You can google to see if I'm wrong, but I doubt that it makes any difference where the possession/use occurs.

