I'm sure there is a lethal dose, but it's probably hard to achieve it. If I wanted to die by cannabis, I'd probably distill it into a more pure form like bubblehash, then eat it. Not sure if it'd be possible to die that way though, as you might just pass out during the eating and be unable to continue. But if you set it up as an IV, maybe it could be done, by pumping the near-pure chemicals straight into your circulatory system.