How do you politely drift away from old friends that just don't seem to get the hint?

I have moved on in my life and many of my old friends were not very good to me in the past or they just remind me of times I'd rather not re-live. I have tried not calling them, not answering their phone calls, etc. I don't want to bluntly say anything to them because I don't want any bad blood between us. I just don't want to really be as close as we once were. Anyway, they don't seem to be getting the hint. They still call me constantly. How can I successfully drift apart from these people without having to confront the situation head on. The reason I don't want to do this is because we run in the same circles. I don't want to have beef with them if I happen to run into them. I hope this makes sense.