1. Via the UN security council (yeah, a useless lot the bunch of them) organize boycott (and an embargo?) of Iranian oil export. Yes, the price of oil will sky rocket but offer financial support to participating nations to ease the pain.
2. Do not place geneneral economic sanctions against Iran otherwise as this may hurt the average Iranians. We have seen what happened in Iraq.
3. Engage and support western culture and democratic ideals in Iran to prepare the population for regime change.
4. Support and fund dissident groups in Iran to destabilize the current regime.
5. If Iran deploy nukes, then use conventional weapons to destroy their capacity for military actions. Then invade. Unless France strikes first that is.