unfortionatley, pepermint srimp are notorious for this behaviour, so sorry.
Before I bought the nem, I specifically asked if there would be a problem and was told no. I even read about a situation like mine on wetwebmedia ... the article said that it can happen, but it's not a common occurance. The article, unfortunately, didn't mention how the conflict ended, whether the shrimp stopped harassing the nem or if he was removed from the tank.