Prepare ziti according to package directions. Meanwhile, in a Dutch oven, melt butter. Stir in flour until smooth; gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Reduce heat; add the cheeses, garlic powder and pepper. Cook and stir until cheese is melted.
Drain ziti; add to sauce mixture. Stir in ham and spinach. Transfer to a greased 13x9-in. baking dish. Bake, uncovered, at 375° for 20-25 minutes or until heated through.