what should I do about these people?

I hate people who are pressuring me 24/7 to give my baby formula. I breastfeed him and I know it's enough for him because I pump 1 and a half bottle and sometimes 2 bottles for him. But am tired of people telling me to give him formula and that he will grow up fast and if I am doing formula and breast milk together. 
How do I answer them? I want to tell them to fuck off in a nice way. Thank you