Just tell them - "shut the F$ up"... no seriously, here is what

when I hung out with a few friends (americans) at lunch with me being the only Chinese. Sometimes people pick on me when another news such as another china-made tooth has lead in it. Guys sometimes trying to be funny and asked me what's going on with you chinese people. If I am in a good mood I would play along saying "hey this is another round of terrorist attacks on Americans from China...". If I am not in a good mood, or if they cross the line, I would just tell them "hey you know my chinese, shut up or I will throw my lunch at your face". People are all smart. They all understand certain boundaries cannot be crossed.