Tell them "thanks for your input, however this is what I have decided is best for me" then move on. That doesn't mean you shouldn't consider their ideas - just ignore the words as some people have never bothered or can't for whatever reason practice diplomacy.
If you are a writer - you're going to deal with criticism a lot. Learn from it, don't take it as a personal assault, and then move on. Be polite in return and be the bigger person. If a person is just being negative and you react negatively in return you just feed the troll ;)