Preventing outside heat from hitting cabinets
You could add a layer of rigid insulation to the back of your cabinets, but you'd lose a couple of inches of space and you'd want to add a covering over the insulation. Plus, I doubt it'd do much (for all the effort), because the heat is already in the cabinet. 

Likewise, removing the cabinets and installing insulation in the wall seems expensive...and maybe the wall is already full of insulation. 

The best way to keep heat out is to prevent it from entering the wall...how about building some latticework outside and planting some vines to cover it. The trellis should be a few inches away from the wall to be most effective. (I suppose you could be really creative and build an insulated wall with latticework on it to really "protect" your wall.)