I would tend to agree with the perspective advocated by Peace on this. An alternative would be for the US, 'unofficialy', to let Tehran know that should they pursue a policy that would lead to them acquiring nuclear weapons that the US would allocate on a full-time basis three SSBN's tasked specifically with the defence of Israel, thereby providing Israel with an invulnerable second strike capability.

With three operational SSBN's it would not matter how many nuclear weapons Iran had, because any use of such a weapon against Israel, directly, or a-symetrically by Iranian useage of any one of the numerous terrorist groups it supports, would result in the total destruction of Iran. Actually I do not believe that they would have to go that far, the cities of Tehran, Bandar Abbas and Quom would be sufficient.