This rune spell was developed to hinder men getting an erection. You can rewrite the text so that he will only get it up with a specific person, but fails with everyone else.
Get a link of the target. A full body picture would be best for this.
Place a penis shaped candle on your altar. An ordinary black one would work too as a substitute.
Create a bindrune of the runes:
Declaration of your Statement of Intent
Light the candle
Place the picture of the target between you and the candle.
Start drawing the bindrune on the targets groin (each rune separately), with each rune, say a short galdr.
When you're done, visualize the power of the runes merging.
The groin chakra can be visualized as a spinning vortex that is being closed when you evoke Isa. Also visualize the energy leaving there when evoking Berkana and Ingwaz.
Finally place the burning candle on the photo and let it burn down.
Banish with laughter