I frequently have adult Balls around my shoulders/neck. They're not very strong and even if they did "try" to constrict someone, the most you would have to do is uncoil them. The fact is, though, that they would probably never constrict someone's neck other than to hold on and feel more secure. Certainly not a feeding constriction.
I think the only way a Ball Python would be considered dangerous is if you trip over one and fell down a flight of stairs.
As far as what to do if it won't let go? It'll eventually let go. It's just a Ball Python and once it realizes it's not in any danger or can't eat you, it will relax its grip and let go on its own. You could try one of the tricks like running water over their head, but it's not necessary.