There are fewer options for boys who can’t get the GnRHa medicines. Luckily, starting testosterone therapy to induce male puberty is often enough to suppress ovarian function.
Drugs such as raloxifene have been used to suppress the effects of estrogen in other conditions such as unwanted breast growth (gynaecomastia) in cisgender boys. There's little data available about their use in trans boys. However, there's no reason they wouldn't work in the same way to prevent the effects of estrogen. Raloxifene works to prevent estrogen developing the breast and womb, so it will help suppress breast growth and periods.