Wow...He is at least trying to say Hi even though the friendship is over. Have you asked him to stop being friends with his group of friends, or just simply leave whenever you're around?
What you ask of him is too much, if you want him to stop talking to you, just tell him "Please stop talking to me". It almost sounds like you don't want this person around you or your friends at all. If you're "indifferent" to someone, then you usually don't care what they do or if they say anything to you. You still hate this person for one reason or another and don't want to sound petty admitting it.
If you don't want them to acknowledge you, then simply ask them to stop. This will however require you to spend some of your precious energy talking to them.
"All is fair in love and war." - Francis Edward Smedley