The offence is preventing the keeper from releasing the ball, or attempting to kick the ball as the keeper is releasing it. They can stand in front of the keeper, but if they then move with him the referee has to decide if the keeper is able to release it fairly. I usually shout "don't block him" and they move off. If the attacker is very close at the time of the release, and the ball clearly doesn't go anywhere near where it was intended, and the keeper is moaning, the safest option is just to give an IDFK.