tbon, I think that alot depends on the reason that you want a dick to stay limp. Perhaps if you could give us a bit more detail, we could give you more help.
not sure about long term side effects
but liqourice causes imotence, and is natural, the better quality the sweet, the better the effect, or 'water tablets' that you can buy over the counter, but take a few days to have that effect, or alcohol, or old age, or that guys mother in law
Okay this might be something that you want to ask on the bdsm board about then. Because I really am at a loss for any suggestions other than the ones that you have already been given.