Broad was found guilty of dissent in breaching the International Cricket Council code of conduct on day four of the second Test against South Africa.
The incident happened after the umpires had warned England captain Alastair Cook about Broad damaging the Newlands pitch by kicking it in frustration.
The 29-year-old had denied the charge.
Match referee Ranjan Madugalle conducted a formal hearing after Tuesday's play in Cape Town.
The incident occurred in the 195th over of South Africa's innings as the hosts compiled 627-7 declared.
Broad was found to have breached the ICC Code of Conduct for "showing dissent at an umpire's decision during an international match".
England fast bowler Stuart Broad has been fined 30% of his match fee after twice telling umpire Aleem Dar to "just get on with the game".