I would ask for the manager, it seems very unscrupulous of the waiter/waitress to immediately add the tip.
The typical tip is between 15 and 20 percent, but if you didn't like the service, there is no requirement for you to pay the gratuity.
I would do as someone else mentioned and check to see if it is posted that all checks include the gratuity, and if not, request that be removed from your bill.
I also would not return to that restaurant, there are so many that will respect you whether you are African American, white, or green.
I once got bumped at a fast food place of all things, because the employees were all African American, the other "guests" were also, and I was the only white woman, with a Hispanic hubby.
We saw our food sitting on the counter in the kitchen area and they just let it sit there, while they served others who had come in after us. I asked for my money back and we walked out. They tried to say "Oh, but your food just got finished" but I pointed at it, and said "It's been back there for fifteen minutes."