Tamoxifen

Tamoxifen is a SERM (Selective Estrogen Receptor Modifier). It modifies certain estrogen receptors (in this case, in the breasts) and leaves other alone (it acts as estrogen in bones). It is often used for breast cancer treatment for this reason, as it prevents osteoporosis and the spread of estrogen-sensitive cancer in the breasts and can be combined with GnRH blockers (either GnRH antagonists or agonists) for treatment.

In other words, tamoxifen acts as an anti estrogen in the breasts but as estrogen in bones and skin.

This medication would likely be effective in halting breast growth (as it would prevent estrogen from acting in the breasts), and is already used for the treatment of fibrocystic breasts and breast cancer due to lack of side effects compared to other medications like danazol. It is sometimes used in treatment of gynecomastia.

It can possibly lower levels of IGF-1 (insulin growth factor 1), but also can lead to higher levels of estrogen in the uterus, which could lead to uterine cancer (though the risks are small).

I would recommend going with raloxifene instead (another SERM), as it is does not increase uterine levels of estrogen, and has a greater affinity for increasing estrogen levels in bone.