Coat a large heavy-bottomed pot with nonstick cooking spray. Add ground beef and cook over medium-high heat, stirring, for 5 minutes. Add onion, pepper and garlic and cook 4 minutes. Stir in chili powder, oregano, cumin and cayenne. Cook 1 minute.
Stir in diced tomatoes, tomato sauce, sugar, tomato paste and salt. Cover and cook over medium heat for 10 minutes, stirring occasionally. Uncover and stir in beans. Heat through, then serve garnished with sour cream and scallions, if desired.