Sorry for bumping a really old thread but this is what came up when I was googling "anti erection." I want to tell you an interesting way to control erection. It's only a shame that my advice doesn't help my situation! oh well!
Engage the prostate by sitting on something with a round flat top and bottom, a spray can, shaving cream can, a cup, something tube-shaped and won't break. Anything that has this shape:
http://store.iuneeds.com/images/gillettefoamy.jpg (image is of a gillete fusion foam shaving cream). I think a large rubber ball (superball / bouncy ball) might also be able to put enough pressure and may be more comfortable.
You want to sit on almost like it's trying to penetrate you. You don't need your full weight. You can move it forward closer to the scrotum away from the anus as well. When you get it right, you'll feel a powerful sensation, a heartbeat in your prostate, or maybe it will be less powerful depending on how much pressure you are applying. Anyway, sitting on it with the right pressure will cause your penis to deflate. Variable pressure may allow you to get any level of erection. Since this is pleasurable you can get fully erect with less pressure or fully flaccid with more.
Side effects include horniness, extreme penile pleasure sensitivity, extreme desire to ejaculate, and in rare cases also cause prostatic fluid to be pushed into the urethra. If you get near orgasm and even go "over the edge", stop all stimulation immediately! Applying a good amount of pressure will stop the orgasm dead in its tracks, you will be able to feel a little bit of the orgasm as it gets halted. This is actually a great edging technique. However, if you are going over the edge and continue to stimulate the penis, you will go fully over the edge and retro-ejaculate into bladder unless you release pressure.
Experimentation is required.