The campaign, which launched at the end of last year, is for a nasal spray that claims it can delay premature ejaculation.
The Advertising Standards Authority said the ads must be taken down as the product is a prescription drug.
The ASA is also investigating the campaign after receiving 450 complaints about the 200 posters.
In response, AMI Europe's medical director Dr Michael Spira said the company would not be taking down the posters and described the ASA's action as "bizarre".
Spira said: "Censoring ads that prompt men to take action about an important health issue should not be taken until a full investigation has been completed."
AMI is co-operating with the investigation process.
The ASA stated prescription-only medicines cannot be advertised to consumers and said the campaign needs to be removed in the interest of public safety. It has the power to remove the ads if AMI refuses to co-operate.