If you use chloroform, that gas that knocks you out, or something else that isn't harmful, it's fine... but if you use your fists and beat them unconscious, you could very well cause brain damage. You could be responsible for ruining their lives, when it was remotely possible that they might have gotten home without an accident. I don't think a blow to the head is okay.
I would support your decision to disable their car by any means necessary, though. Remove the battery, cut wires, cut the fuel line, slash their tires, etc.
Anything that knocks you out is potentially deadly/damaging to the brain, including chloroform, its not like it shows on tv, its very easy for a person to OD on chloroform and die... your idea of disabling their car is a much much better one, I am not even sure how much trouble you could get in for it, as you are keeping some one from committing a major crime that is potentially deadly to others... and doing so in a way that is not directly harmful to them, but Im sure we can all agree here, the BEST case scenario would be to grab the keys and hide them somehow... not to hard to do from most drunk people ive ever dealt with, defending against their usually violent response to that is another matter...
"How dreadful!" cried Lord Henry. "I can stand brute force, but brute reason is quite unbearable. There is something unfair about its use. It is hitting below the intellect." ~ Oscar Wilde - The picture of Dorian Gray
the cops are not useless. Just make sure you call them when he is NOT behind the wheel and don't tell them that he's been driving after drinking. Also make sure he is in a home/apartment, not in public. The cops will either scare him out of driving, or restrain him since you are not able to.
Really it's not worth facing possible assault charges, brain damage, or the drunkard recovering and getting violent with you. That is what law enforcement is for.