Lowering your testosterone low enough to stop all erections for a year or two with some low dose estrogen is about the only way to do it.
Prolonged chastity in progressively smaller cages can do it to, I fine lady posted some pics of her work a few days ago, check them out.
Ecstasy does it well. I have a large cock but Goddess feeds me Ecstasy to shrink it away to nothing. From wearing the Curve I go to wearing the Ultra Smalll