Can breast cancer be prevented?

Yes with surgery. Mastectomy of both breasts will be recommended to prevent the development of breast cancer in patients who carry genetic abnormality such as brca 1 or 2 as this genetic abnormality will increase their risk to have breast cancer and ovarian cancer significantly. Tamoxifen is given to patients with high risk feature like precancerous /in situ to reduce risk for recurrence of in situ or cancer.
Risk can be reduced. For patients at very high risk (very strong family history or known genetic mutation), bilateral prophylactic mastectomies are an option. Taking Tamoxifen can also cut the risk of breast cancer by half. For the typical woman, the risk of getting breast cancer can be reduced by maintaining a healthy body weight, getting regular exercise and eating a more plant-based diet.
Some cases. Unfortunately, we only have limited knowledge of the specific causes of breast cancer, making it difficult to identify methods to prevent it. Nevertheless, women at high risk for breast cancer may take tamoxifen for "chemoprevention": 5 years of therapy may reduce their lifetime risk of developing breast cancer by as much as 50%.
Risk reduction. Tamoxifen and Raloxifene have been shown to reduce the risk of er+ breast cancer by 40-50%. Ais more recently (study of exemestane) increase this rate to 65-75%. Chemoprevention has side effects to be weighed. Prophylactic mastectomy is >90% effective in risk reduction but is a very aggessive intervention most appropriate to know mutation carriers (brca 1/2) with a lifetime risk of 70-90%.