Cook macaroni according to package directions; drain and cover to keep warm.
In a large skillet, brown the ground beef, bell pepper, and onion over high heat for 6 to 8 minutes, or until no pink remains in the beef, stirring frequently. Drain off excess liquid.
Add macaroni, spaghetti sauce, garlic powder, salt, and black pepper; mix well. Reduce heat to medium-low and let simmer 5 to 7 minutes, or until heated through. Sprinkle with cheese and simmer 1 to 2 minutes, or until cheese is melted; serve.