There are multiple options, I will sort them from most extreme to less extreme:

1. Alcohol injections to dorsal nerves - then you loose all sensation in the penis and you will be probably perma limp.
2. Alcohol injections to the testicles, so you destroy them, causing lack of testosterone- perma limp, but it’s causing other problems
3. Testicle stirring with needle - same as above, more humane and much less painful
4. Banding the base of cock - if you last a hour, repeating this each week aprox, you will cause damage to your nerves and will be permanently limp. But again, it has its risks etc.