How do stop racist nickname at work? ?

I find it hard to believe any HR person would tell you what you say they told you.

You need to stop them once for all. Either you do it in public or private but unless you show that you are not an easy-going person, they will keep harassing you.

Better confronting now than later!

There are three paths.  You could either respond by calling anyone who calls you that a mother lover. Or you could quit and tell HR that you have been constructively fired because of discrimination.  Or you could stay and file a lawsuit, but you had better have good records and tapes of actual conversations and when they happened. 

you can try suing the company.  record it on your phone while at work.  hire an attorney.  

Experts encourage employees to keep an updated Resume and always have a back up plan.  You become a target whenever a boss or coworkers feel like you don't have any options.  Once it becomes clear you can't leave or won't leave no matter what they do to you, unfortunately, you lose all bargaining power.  Like you said, going to HR is somewhat pointless in that type of culture especially if prior complaints have been lodged by fellow employees with no results.  Plus, you can expose yourself to retaliation.

It can be hard to leave a job and start over at another company when you have been made to feel unvalued.  But, remaining in that environment only enforces your lack of self worth.  You deserve to feel valued and have your contributions appreciated.  You may not be able to control how they treat you, but you have 100 percent control over how you treat.......or mistreat.....yourself.  Sometimes leaving a bad job or a bad situation is the best thing you can do for your health, peace of mind, and happiness.

do not feed the troll 

And you are a troll!