The Advertising Standards Authority (ASA) said Young's broke the advertising code of practice for two poster adverts featuring a man with a ram's head: one with him surrounded by several gazing women in bikinis and the other with him entertaining men in a gentleman's club.
Both adverts carried the slogan "This is a Ram's World". Young's has used a ram as the logo for its brewery for more than 150 years.
The ASA ruling makes Young's the first drinks company to fall the wrong side of new regulations for non-broadcast alcohol adverts, introduced on 1 October last year.
The ASA said the ram in the first poster personified Young's drinkers and that "by showing the ram as the focal point of the attentions of several women, suggested that Young's drinkers were more likely to be the target for seduction".
The Authority said the second poster also broke the code because it linked alcohol with social success. It has told Young's to withdraw both adverts.
The brewer had argued to the ASA that the ram was not intended to personify its target audience and that neither poster showed the consumption of alcohol.
The ruling is a sign the ASA is prepared to get tough on alcohol advertising under the newly tightened regulations, reflecting government concern over Britain's binge drinking problem and also underage drinking.
The new rules, as well as banning links between alcohol and social or sexual success, also forbid linking alcohol with irresponsible, anti-social, tough or daring behaviour, showing alcohol being handled or served irresponsibly and so-called "kidult" advertising - showing drinkers behaving in a juvenile way or "reflecting the culture of people under 18".
The Committee of Advertising Practice (CAP) has already highlighted the part on under-18 culture as something that would be difficult to police.
"We try to give as much guidance as we can," said an ASA spokesperson, adding that it was impossible to cover everything because there are "so many different scenarios that can happen".
The CAP published research just before Christmas on young peoples' drinking habits and what types of adverts generally appealed to different age groups, as a way of helping the ASA make decisions.
It admitted that adverts bringing the teenage and adult worlds closer together "appear to have strong appeal for young people".
The ASA spokesperson said the Authority would assess the whole feel of the advert instead of focusing on different aspects individually. Drinks makers can also obtain free advice from the ASA before going ahead with adverts.
The British Beer and Pub Association declined to comment on the impact of the new non-broadcast advertising rules.