1. Destruction of property (smash your car).
2. Physical harm (smash your head).
3. Psychological harm (stalking someone).
We already have laws that cover this stuff. We have plenty of settled cases that show what is and what isn't harmful. There's always going to be room in the law for some wiggle room - and that, I think, is okay. Every situation is going to be different (which is why mandatory sentencing laws are so terrible).
If someone wants to rot their brain with a kilo of coke, well, I don't approve but I'm not going to stop that person as long as they don't present a danger to life, liberty, or property. Want to squirt some krokodil into your veins? Have at it. Enjoy your short life. Just don't dick with me.
That actually may sound cruel; after all, we know that these drugs do serious damage to a person, and we could prevent that person from hurting themselves... but, little by little, as we have seen, it becomes, "drinking that beer is harmful, you can't do it" and "reading that book is harmful, you can't do it."
The best solution is somewhere between "go snort yourself to death" and "obscene nanny state"; you can tell which side I lean towards, obviously. (o: