When a person is diagnosed with ankylosing spondylitis, will they become deformed? is there a way to prevent the bent over posture with exercise?

Not necessarily: While making sure you are exercising and optimizing your flexibility and core/back strength/tone, optimizing your ergonomics/posture, you need to work with your doctor(s), to make sure this inflammatory condition is controlled and that your bone density remains optimized. Remember the spectrum of AS is quite variable, some people will have very mild symptoms and issues other more severe.