Bad drivers!!!

So lately I've come across some real bad drivers on the road, take 3 weeks ago for example.
I was driving up the road and as always, I observe other drivers around me. There was a guy infront of me swerving over to the other side of the road, ok, I thought it could have been something on the road, but then he gradually drove slower swerving all the time and I realised he was then drink driving.
I don't know what the right thing to do in these situations so I was flashing my lights and beeping at him, tempted to follow him untill he stops and have a word, so I followed him a little further untill I saw a cop car pull out and that was that.

Today on my way home from College I was following some rather young girl driving a Citreon C2 and she was going over the other side of the road, had a passenger in the car and a 'baby on board' sticker in the back, She made a slight swerve and of course I didn't take much care of that, then she was swerving over to the other side of the road and shaking the steering what seemed to be on purpose.

What do you guys do in situations like this?