The ad, created by Contagious, featured a man speaking to his boss who responded angrily and loudly in German. The voice-over said: "Boss a bit of a tyrant? Find your perfect boss on the UK's biggest job site...".
After being aired last September, 13 people complained that it was offensive to Germans, because it used an outdated stereotype and implied all Germans were tyrants.
The complaints were upheld by the ASA, who ruled it had breached codes of good taste and decency.
An ASA statement read: "We noted the ad used a German speaker, rather than someone speaking English, to portray the boss as a 'bit of a tyrant' and the humour derived from a stereotype at the expense of German people.
"We considered that the portrayal suggested that German people were more likely to be unreasonable or aggressive to others."
Today's ruling will be an embarrassment to industry approval body the Radio Advertising Clearance Centre, which had considered the ad would be "humorous and inoffensive" to most listeners and acceptable to broadcast.
The association responsible for checking the content of prescribed radio ads and sponsorship credits in the UK argued the character was a generic "German-sounding orator", which it believed was a well established type in comedy culture.
The RACC did not regard Germans as a minority group, as defined by the Cap Radio Advertising Standards Code, or that the scenario would be seen as a stereotype likely to cause general or serious offence to German people.
Brands have long leveraged German national stereotypes held in the UK.
In 2005, a Carling Black Label TV campaign by Leith London played on the historic Dambusters run when the British dropped bombs in Germany.
The creative culminated in a German guard on the dam assuming the role of a goalkeeper and saving the dam by stopping each one of the bombs.
A follow-up ad featured a British tourist throwing a Union Flag towel which skipped off the water like a bouncing bomb to reserve a pool-side sunbed before the German tourists could reserve them with their towels.
Both actions were followed by the beer brands strapline: "I bet he drinks Carling Black Label".
Shepherd Neame and its agency RPM3 similarly ran war-themed ad campaigns to promote its Spitfire ale on terrestrial and satellite TV, and via underground posters, which were reported to the ASA but not banned.
German brands have often used the positive national stereotypes associated with Europe's biggest country in their ads, including Audi's 'Vorsprung durch Technik' and BMW's celebration of its engineering.