Can coronary artery disease be treated?

Certainly. Evaluation by a primary care & cardiologist, reduce weight - BMI < 28, eat right: low fat, low (bad) cholesterol, consider high quality fish oil, cessation of smoking, exercise (if you are healthy enough per your physician), reduce alcohol consumption, take prescribed medications as directed. Know your numbers: cholesterol, blood pressure, blood sugar and work with your physician to optimize them.
It depends on. One's understanding of the word "treated". If one means "cured", the answer is no. If one means "prevented", then not totally. Currently, treatment modalities are medical and procedure-based. Medical rx relies in control of diabetes, hypertension, dyslipidemia and smoking; includes Aspirin and/or plavix (clopidogrel). Procedure-based rx can be plasty/stent or bypass.