Even the most tame and intelligent ray will likely freak out if you apply that amount of force to it's natural defense mechanism. Really worth getting stung over? Sure you can pet and hand feed some rays, but do I really have to reference Steve Irwin here? Leave well enough alone, if it's a seriously problem then you have to be prepared to deal with the consequences (remove your other fish, get rid of the aggressor, or stress the ray out by taking it out of the water and SAFELY remove the barb). You ever been stung before Charney? I haven't, but from what I hear I don't want to.
Maybe if you had a large pond with a large amount of older rays that are used to a vast amount of people coming and touching them (they likely come to you expecting food so I get your logic) your idea would work, because I can see a public aquarium doing this, but I really think this is probably only because the keepers can easily get into the water, and the rays are so used to it that they really don't mind, likely because of how many times it was done to them, and the fear of people being almost entirely wiped clean because of their exposure to people. I say almost, because I am still willing to bet the odd ray freaks out. I think it would be much much harder to tame a couple leo's in an aquarium like you would rays in a "petting zoo". And by the time you tame them, you would think you wouldn't need the barbs cut.