First, the Rikti would have to come to the conclusion they cannot win their war here on Earth.
Then representatives from the UN Security Council, Vanguard, the US State Department, Paragon City officials, Statesman (secretly Tyrant), and many others would have to sit down with Rikti representatives and sign peace accords.
A plan to dismantle the walls, while keeping some sort of safeguard in place, would need to be drawn up. Funding would need to be secured. (I imagine a reasonable tax on Wentworth's might be enough.)
Then construction contracts...
It would be a lot of effort.