Under the Laws of the Game, coaches must behave responsibly. Technically, they do not commit misconduct under Law 12 and cannot be shown cards. However, most of our leagues permit, or even require, that coaches are shown cards when their behavior becomes unacceptable.
In dealing with coaches, USSF recommends an Ask, Tell, Dismiss approach. First, ask the coach to refrain from the unacceptable conduct. Second, tell the coach to refrain from the unacceptable conduct. And third, dismiss the coach.
It should be noted this is a recommended approach. A coach who commits an egregious offense can be dismissed immediately. Nor does a referee have to go with a "three strikes and you're out" approach. Youth referees, particularly, are at a disadvantage when dealing with adults. A young referee may hear something and ignore it, once, twice, maybe even three times and then show a red card, catching the coach off guard.
In all cases, the coach is to behave responsibly. Whether or not a referee follows this approach does not change the fact that a coach can be dismissed for irresponsible behavior, with or without warning.