Male goat fertility
At what age does the fertility of a dwarf goat buck end so he can safely return to the herd?
Spartanburg County South Carolina goats
If the intent is not to breed any of the does...
Buck fertility will generally decline with age, though there is no specific age at which they will no longer be fertile. I would recommend castration, by your local veterinarian (especially if the buck is older), as the most effective method in preventing pregnancy. Breeding soundness exams, also done by your local veterinarian, can give you a glimpse at the bucks fertility, but will not necessarily guarantee that he cannot breed the does now or in the future.
If the intent is to avoid aggression...
Bucks become aggressive due to their hormonal makeup as they age. This is especially true during the rut. They have developed this to fight off other competing bucks, where the winner gets to breed the does. It's in their nature. Some goat owners believe that behavioral training and establishing dominance may decrease their aggression towards people. Castration may also be a good option as well (if you don't need him for breeding purposes). Depending on the bucks age, this may not affect his disposition. The best way to prevent aggression (in non-breeding males) is to castrate them young.