You can't, not even when you tell them where the rumor came from. I've heard white liberals tell me the same thing. The AIDS hokum and that Central American children were being grown for organ harvesting by rich people both have the same origins. They were "parting gifts" from the KGB. In the mid 1980s the KGB started both disinformation campaigns at about the same time and kept them running till they closed up shop. They worked well on morons.
I had no idea that's where those stories came from. It's just very very difficult to fight someone's world view. If you grow up thinking that most white people (and all republicans!) hate you because of your color, then it's easy to believe that kind of stuff.
susie