The problem with lots of the idead is that they will involve killing turtle, which will prevent him from suffering enough.
I would recommend the following: Build a retracting bridge 10 tiles long (perhaps two end to end). At the end of the bridge, build a lever that is linked to this bridge. Underneath the bridge(s), there should be a drop of about 5 z levels - enough to injure him, but not kill him. Make sure that the path to this level has a pressure plate on it to reactivate the bridge(s), so when he gets better he'll go straight back to the lever.
Alternatively, you can do something else. Build him a hut outside the entrance to your fort. It should look like the following:
M=magma
W=water
/=window
_=channel
/M/_ /M/_
WW
He would live out the rest of his life in a 2x2 room - One square of it would be a grate over a water source, and the other three would be a farm plot, where he can grow his own food. The magma is symbolic of his eternal burning in hell. THe glass windows are so any hostiles or animals outside scare him. The channels on the outside of the construction are to avoid building destroyers destroying the window.
Alternatively, you can encase his entire family inside. All his offspring will be trapped inside for all their life. Although they should increase in numbers at a rate of about one a year, the limited food source means that if the numbers get too large, some will die off, providing bad thoughs and a miasma. Ideally, you want turtle to be constantly depressed, but not so depressed as to off himself. If any dwarves in the fort get involved with the offspring of turtle, then they can be placed into the construction my means of restracting bridge, so turtle can have his grandchildren subjected to the cell.