There has been a long time story about a food additive called Saltpetre, which has neither been conclusively proved or disproved.
The story may only be urban myth and usually goes something like this: In the past, various military organisations have used the additive Saltpetre in the food that it feeds to it's personel. The purpose of this additive was to significantly reduce or prevent the occurrence of erections. This was intended to prevent sexual activity between the personnel or with civilians.
The chemical compund Potassium Nitrate (chemical formula KNO3), also known as Saltpetre, has long been thought to decrease sex drive and more specifically to prevent erections. There does not appear to be any specific scientific evidence to support the claim that the substance causes such an effect, however there does not not appear to be any real research done on this aspect either. Potassium Nitrate has been long used in food manufacturing due to it's usefulness as a preservative. Potassium Nitrate also plays some role in the oxidation process, as in salami which remains pink, the meat would ordinarily turn brown without it's presence.
I do not know if Potassium Nitrate (Saltpetre) really prevents erections, however it is well known that both Sodium and Potasium in the diet have an adverse effect in the human body of increasing the blood pressure, and increasing the blood pressure has an adverse effect on the ability to get an erection (ie. the ability decreases).
Why not try hypnosis?
BTW, HNO3 is Nitric Acid I believe, and not Saltpetre. It probably won't prevent an erection, but it probably could burn you penis right off!
-= ... mch ... =-