Here's the laws where I live from the Arizona Boxing Commision, so it applies to fights IN Arizona only.

Historical Note
Adopted effective January 21, 1981 (Supp. 81-1).

ARTICLE 3. CONDUCT OF CONTESTS

R4-3-301. Fair blows and fouls

A. The only fair blow is one delivered with the padded knuckle part of the glove on the front or sides of the head and body above the belt.

B. All blows that are not fair as described in subsection (A) above are fouls. The following practices are also classified as fouls:

1. Hitting an opponent who is down or in the process of getting up after being down.

2. Holding an opponent with one hand and hitting with the other.

3. Holding or maintaining a clinch after directed by the referee to break.

4. Pushing or wrestling.

5. Butting with the head or shoulder.

6. Hitting on the break.

7. Hitting after the bell has sounded ending the round.

8. Any unsportsmanlike trick or action likely to cause injury to an opponent in the opinion of the referee.

9. Refusal to obey the commands of the referee.

10. Falling down intentionally.

Source: http://www.azsos.gov/public_Services/Title_04/4-03.htm