Can you prevent pubertal gynecomastia from happening with letro or something else?

NO: If you are going to develop you will develop gynecomastia , no medication will prevent it , if psychologically bothering you get fixed by surgery which will give normal appearance.

Not Necessarily...: I am not aware of any medications useful in the prevention of gynecomastia. Letrozole, an aromatase inhibitor, as indicated in the treatment of breast cancer/tumors. Be careful about the source of information you receive; there is a lot of misinformation around, for example on the bodybuilding blogs etc.