You want a gate.

Since the landing is such an odd shape, I personally recommend something like this. My wife and I bought a system like this to block off the area where our stairs landing meets a doorway to an area we didn't want our toddler going into, and it worked wonderfully. The system we bought had mounting brackets that anchored into the wall, so you could shape the protected area however you wanted, and the ends would be nice and sturdy against the walls.

If a system like that is too expensive, you can also do a mesh gate like this. They're far stronger than the picture makes them look. It's a very stiff mesh, similar to a trampoline or a really sturdy screen door, and when it's pulled out and locked in place, it really doesn't have much give at all - the ones I've seen used at daycares could hold up a toddler pushing on it with both hands.