A complaint was filed with the ASA claiming the ad was irresponsible because it might encourage children and young people to gamble.
The ASA also challenged the ad as contrary to a Committee of Advertising Practice code which states no one under 25 can appear in a gambling ad in the UK.
Betfair argued it did not target its marketing at under 18-year-olds, claiming the ad was only emailed to people over 18 and it implores technical measures to prevent children and young people from gambling on the site.
After hearing argument from both sides, the ASA released a decision Tuesday ordering that the ad must not appear again and Betfair must ensure no one under 25 appears in its ads in the future.
"We concluded the ad was likely to have particular appeal to children and young people, and that, by representing a successful young poker player with the implication she was 15 years old, it could encourage young people to gamble and was therefore irresponsible," the ASA's decision reads.
Betfair argued that Obrestad, 20, was featured in the ad not because of her age, but because of her standing as a poker professional who has had a profound effect on the game.
However, the ASA decided her appearance in a gambling ad was still in contravention of the Committee of Advertising Practice code.
Despite claiming it would effect Obrestad's ability to derive earnings from sponsorship deals, Betfair said clear instructions had been given to its marketing department not to use her again in marketing material directed at UK consumers.
Obrestad built her bankroll by playing with the online handle "Annette_15", an alleged reference to the age she started playing.
Obrestad was 18 when she shocked the poker world by winning the inaugural WSOPE Main Event in the fall of 2007 for over $2 million.