It is nearly impossible to stop true moles from forming. However, if the pigmentation in question is not made up of moles, but rather a solar lentigines or "sun spots," you can lessen your chances of developing more by limiting sun exposure. Make sure to wear a broad spectrum sun screen every day, and if you are out in the sun for extended periods of time, reapply the SPF every 2 hours at least. If the pigmentation on your face is made up of sun spots, you could use a product with bleaching agents to lighten the spots. I would suggest you schedule a visit with a dermatologist to identify the lesions on your face, and discuss treatment options.
Best of luck!