I'm afraid the only answer I can give here is that you can't. Since the reward is coming from someone other than you, that person will have to stop providing the reward. You may find the child providing the reward doesn't want the interactions to stop. If this is the case, the only possibility for you to gain control yourself is to use punishment. As I have said many times before, I prefer not to use punishment except as a last resort.
Punishment gets to be a pretty sticky issue when bickering and fighting is going on.
Remember my story about my little brother kicking me and me getting punished for whacking him back. He was rewarded by my punishment, so that didn't work. A parent's sense of justice probably won't allow punishment of the innocent-appearing party along with the guilty one. I don't think I would expect that to work anyway. They probably both deserve punishment anyway.