How can you stop people from being racist?

You can try to calmly let them know that you are not comfortable with some of the statements they say and ask them if they would mind avoiding saying them when you are present.

Though racism is obviously wrong, some people simply refuse to understand that. You can't shout at someone that they are racist, bigoted, etc and demand that they stop. There is freedom of speech and they can say what they wish.

But this person may not realize they are hurting your feeling or making you feel uneasy around them. If this is a person you work with and they still will not stop after you ask politely, you have the option of going higher up (boss, supervisor, etc) and let them know that you are uncomfortable with some of the things this person is saying. Your place of work is SUPPOSED to make sure that it is a safe, comfortable environment for you to work in.

Look dont complain because if they know racism irritates you there probably doing it to annoy you so put up with the odd joke or whatever.