I really do wish this study was more realistic. It's not unethical, but people on the Internet rarely get along, unless their not jerks or trolls to begin with. I would guess that being racist isn't exactly trolling, it's just mean.
Now, being friends, it wouldn't really be a problem, cause it's O.K. But 99% percent of the Internet is usually anonymous, and trolls do take that as the upper-hand. Unfortunately, hate spurs more hate, because if Anti-Racist people were to go against trolls, they would be no different.
Being a troll, and using racism as a method of attack, people are going to react, and that is what the troll wants.
If anything, the method would involve that racism is in the past, forget it, or do not react AT ALL.
Also, the 500 racist people, their defined to be racist, and ask racist questions IF they're trolls. Maybe if you have 500 nice people would work, but not exactly what we want...
I'll leave it at that. Thought it is possible, it would be beyond impossible to completely extinguish racism.