With what Willar gave to me, using the ideas of Iluq gifting certain people spiritual powers and placing them at specific points around the world to see everyone at once (in a way), there will be guards and people who regulate traffic between the nations, and keeping tabs on a lot of people in order to find the Avatar. While she easily could stop travel between the nations, there are too many things that would be risked economically if things like trade were stopped because she outlawed travel between nations. And while that doesn't really effect the Nomads, it still hurts the other three nations, including her own water tribes.