Many people have written stories where Harry removes the dark mark from Snape using a ritual, gems, potions or a combined attack of some sort. Since I am rather pants at writing stories to completion, I have an idea that any and all other writers are welcome to use so long as I get credit for the idea.
Step 1: Harry discovers that a severed bodypart does not immediately die when severed from the body. (i.e. Scanned with a life detection spell)
Step 2: Through judicious questions to Snape, Dumbledore, Pomphrey, and Moody he finds that the DM is tied to the life-force of the carrier.
Step3: An unknown (*coughHarry*cough) assailant attacks Snape, cutting his arm off at the elbow. Taking Snape's wand, he casts Avada Kedavra at the twitching limb, severing the connection of the DM between Snape and Moldyshorts.
Step 4: Slapping a ministry issue portkey into Snape's other hand, he tucks the 'dead but still warm' limb into Snape's robe and activates the portkey to take him to St. Mungo's.
That's all I really have for this idea, but maybe sombody will take it and run. This plotbunny is free to a good home. Please feed and water daily.