The ad for Over Farm featured people in monstrous makeup and a cartoon character holding a chainsaw and severed head.
One member of the public, who saw the ad in mid-September, complained to the watchdog that it was unsuitable for display where children could see it.
Primesight told the ASA it had ensured the billboard was not shown within close proximity of any schools, with the nearest one 800 metres away.
Over Farm added that the poster had been designed to make it clear the event was aimed at adults and not children.
But the ASA noted that the billboard was next to a station, and the untargeted medium meant it was likely to be seen by young children, who could be distressed by it.
The ads standards watchdog ruled that the poster breached the CAP Code and ordered that it not be used again in its current form.