Could a 'wind wall' help to reduce the wind shear problem at St Helena airport?
It's a completely different problem, and I'm afraid something like that would make the wind shear close to the ground actually worse since it is inhibiting air flow within 200 ft of the ground while the wind just above is unaffected.

Plus the additional turbulence created by those thingamajigs sticking up.  Most pilots know how unpleasant it can be trying to land on a runway with a stand of tall trees adjacent to it, with the wind shadow and burble generated by the trees.

The windshear problem for aircraft isn't the wind velocity itself, it's the sudden changes in horizontal velocity at certain levels, which in a large aircraft with a lot of mass, creates large airspeed fluctuations as you climb or descend through, along with vertical flows caused by mountainous terrain can can generate high sink rates that, because of the mass, are slow to be addressed with power.  None of that would be helped by a surface barrier.   All the barrier would do is create an extra nasty shear + turbulence right at 200 ft.