The one thing that holds testosterone levels down is running a stallion in with a herd of geldings or other stallion with no mares close by. This has been proven with blood tests. Testosterone levels drop within days of putting them in an all male herd. This is how big ranches get by with running all of their stallions together in the winter with no problems.
Even then, it is not fool-proof. No one should attempt doing this unless they are very experienced and have a really good reason for doing so. But, as a side-bar, it is interesting that hormone levels drop so drastically when they are run together.