Well, there's always the "stop calling/hanging out/speaking method" which I think is pretty uncool but sometimes if the person didn't do anything that warrants a snubbing but you still want to snub, this is the only way to go.
You could always set up an elaborate plan where you lure the person into cheating with someone's boyfriend, stealing, or something equally underhanded, but that may be tough to orchestrate and your friend might not fall into the trap.
ETA: Either I didn't read the subtle part or you edited it after I hit "reply." Elaborate plans are obviously not subtle, but you seem like the scheming type to me...I think you can pull this off.
