Hmmm, a tricky one. My first instinct was to ban religion in the hope that, instead of trying to kill each other over whose god is real/best/most forgiving, humanity actually works together to protect the planet and start us on our journey to the stars. But banning religion would just drive it underground and make everything even more divided.
Banning the use of oil might help, but even though it has the potential to make some countries less of a problem on the world stage, the lack of money could in those same places (cough Qatar, Saudi cough) could also just trigger off a massive wave of killing and death.
Banning nukes is a good call, but it does make me think that someone would hide them and not give a shit about their illegality, kind of like knowing cannabis is illegal, but smoking it anyway*.
The second question is a bit easier for me: I’d transform politics. Make a system that is fair to everyone, leves no one disenfranchised and which cannot be subverted by vested interests or wannabe dictators. Sadly, that might mean the beginning of the reign of Willard the Terrible but, well, you know: omelet/eggs and all that.
* Yes, I know. but you get my point. I could have said breaking the speed limit too, you get my drift, right?