Coral, bet365 and Totesport all sent tweets featuring a photo of the then 21-year-old Spieth in relation to the US Open.
ASA officials challenged whether the ads were irresponsible and breached the code because they prominently featured Spieth, who was under 25 years of age.
Coral marketing said the photo of Spieth was used to illustrate the odds available rather than promote specific bonus offers, but they had made changes to ensure that similar tweets complied with the code.
Bet365 said its tweet was conversational and reported on a major sporting event and therefore did not breach the code, while Totesport said Spieth was neither a young person nor vulnerable and the ad did not show him gambling or any juvenile or loutish behaviour.
The ASA said rules stated that no one who is, or seemed to be, under 25 years old could play a significant role in the advertising. It concluded that the ads were irresponsible and breached the code.