If you want to think about it the exact rule from the playing conditions is:
41.6 Bowling of dangerous and unfair short pitched deliveries
41.6.1 Notwithstanding clause 41.6.2, the bowling of short pitched deliveries is dangerous if the bowler’s end umpire considers that, taking into consideration the skill of the striker, by their speed, length, height anddirection they are likely to inflict physical injury on him. The fact that the striker is wearing protective equipment shall be disregarded.
In the first instance the umpire decides that the bowling of short pitched deliveries has become dangerous under 41.6.1:
41.6.1.1 The umpire shall call and signal No ball, and when the ball is dead, caution the bowler and inform the other umpire, the captain of the fielding side and the batsmen of what has occurred. This caution shall apply to that bowler throughout the innings.
41.6.1.2 If there is a second instance, the umpire shall repeat the above procedure and indicate to the bowler that this is a final warning, which shall apply to that bowler throughout the innings.
41.6.1.3 Should there be any further instance by the same bowler in that innings, the umpire shall:- call and signal No ball
- when the ball is dead, direct the captain of the fielding side to suspend the bowler immediately from bowling
- inform the other umpire for the reason for this action.
The bowler thus suspended shall not be allowed to bowl again in that innings.
41.6.2 A bowler shall be limited to two fast short-pitched deliveries per over.
41.6.3 A fast short-pitched delivery is defined as a ball, which passes or would have passed above the shoulder height of the striker standing upright at the popping crease.
41.6.4 The umpire at the bowler’s end shall advise the bowler and the batsman on strike when each fast short pitched delivery has been bowled. 41.6.5 In addition, a ball that passes above head height of the batsman, standing upright at the popping crease,that prevents him from being able to hit it with his bat by means of a normal cricket stroke shall be called a Wide. See also clause 22.1.1.2
For the avoidance of doubt any fast short pitched delivery that is called a Wide under this playing condition shall also count as one of the allowable short pitched deliveries in that over.
41.6.7 If a bowler delivers a third fast short pitched ball in an over, the umpire, after the call of No ball and when the ball is dead, shall caution the bowler, inform the other umpire, the captain of the fielding side and the batsmen at the wicket of what has occurred. This caution shall apply throughout the innings.
41.6.8 If there is a second instance of the bowler being No balled in the innings for bowling more than two fast short pitched deliveries in an over, the umpire shall advise the bowler that this is his final warning for the innings.
41.6.9 Should there be any further instance by the same bowler in that innings, the umpire shall:
- call and signal No ball
- when the ball is dead, direct the captain of the fielding side to suspend the bowler immediately from bowling
- inform the other umpire for the reason for this action.
The bowler thus suspended shall not be allowed to bowl again in that innings.
EDIT: I've cut some of the unnecessary crap about informing such and such etc.