Tamoxifen is an estrogen antagonist in the breast tissue - it will hinder breast development. Physicians put women who are either at high risk of developing breast cancer or who've had breast cancer, because estrogen affects on breast tissue is known to increase the chance of developing breast cancer.
Men who are taking prostate medication, which inhibits testosterone and increases estrogen, are more prone to developing man-boobs (gynacomastia), so they may be prescribed tamoxifen to prevent that condition, or to halt development. Anyone who wanted to arrest development of breast tissue, at least temporarily, could consider tamoxifen, while considering any known side-effects.