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:

• Uruz (Inability)
• Kenaz (sexual impotence)
• Gebo (sexual blockade)
• Nauthiz (weakness)
• Isa (freezing, blockade)
• Berkana (loss of energy)
• Ingwaz (loss of fertility)

Combined as:

The Ritual

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.

Nauthiz! I evoke your power to create a weakness in <targets name>
Berkana! I evoke your power to drain all energy from the groin chakra of <name>
Ingwaz! I evoke your power to drain all fertility from <name>
Isa! Block the groin chakra of <name> to reproduce energy
Gebo! <name> feels his sexuality totally blocked
Uruz! By your power <name> will be unable to get an erection
Kenaz! By your power <name> shall be impotent

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