To be honest and as a cabin crew member, I have never heard of such a thing. The second advantage of being a flight attendant is meeting different people from different countries and cultures (after going to many places). So having a racist flight attendant is like having a doctor that does not like to touch people! or a nurse that can not see blood. So if there is a racist flight attendant, then she/he must be one of a kind, not something you might see in every flight, not even once in a life time.
Regarding ignoring requests, this is surely not because of racism, this is totally the company's mistake. If a company is enforcing its onboard services, you will never see such a thing. So if this behavior was found in one attendant, be sure it is a company wide issue not a racism issue.
Regarding spitting on food, come on! Again, I do not think this happens. If it really happens, then I guess the rule of "Don't Piss Off People That Handle Your Food" can apply here. Again, from an insider, I never heard any flight attendant ever mentioning this idea, even as a joke.
Regarding fist fight, it is bad by all means (unless it is an act of self defense of course). Flight attendants know how to handle passengers, it is very rare (out of the millions of flights each year) when a flight attendant picks up a fight with a passenger, usually it is started by the passenger. Just avoid such a case, the crew will be filling forms and calling security to be waiting upon arrival, and they know how to deal with these situations, it is almost never a win situation for the passenger. In general, getting physical is a bad idea.
Finally, If you really felt that a certain flight attendant is ignoring a certain ethnic group while serving others, act correctly. Start by requesting to talk to the cabin in charge (purser), explain to him/her what you have experiences. A one or two witnesses to backup your claims and there will be a quick action on the spot! If this did not work then try escalating things and write an official complaint to the company (or sue the company if you have enough proof). Any airlines will not want such a reputation and will try to compensate you so things do not go out of control (media, etc.)