I think an insulin inhibitor agent (if there were such an animal) would basically be diabetogenic. The more useful goal is to boost the insulin receptivity on your cells.. the natural feedback to the pancreas is to then reduce the secretion of insulin because what is already circulating is able to be more effective with the lowering of insulin resistance at your cell membranes.

Cinnamon is one such substance (or rather a compound which they've been able to isolate in cinnnamon) which reportedly has the ability to boost your insulin sensitivity as much as 20x.