There isn't a non-medically dangerous way to do what your talking about. You oughta know that there are ways of permanently losing your ability to get an erection just by common sense. If you are thinking of a medical solution, there isn't one you can get without convincing someone you are a child molester. Then, they could chemically castrate you. But, I doubt that's what you want (considering all the complications that creates).
HOwever, what I'm wondering is why? Dude, permanent is forever. That's a long time to never have an erection ever again. You got problems.
get her to get you birth control pills. Then take 1 pill in the morning and 1 pill at nite. It will take a little time, but, you will lose the ability to get hard!! After all, you do not need to get hard to suck a real mans cock and get fucked in your sissy pussy! Then have her bf's laugh at your tiny clit and humiliate you. That will keep you soft!! good luck!