Hairspray works on fabric and I am told that some ohave had success with it on leather also. If it is the erasable ink type then you can use the pens eraser to emove it.
If it is really set in then you may need to use Leathermasters leather "ink" remover. Use though with caution as it can be some pretty aggressive cleaner.
Some have also has succes with those "magic eraser" pads. So try the least aggressive one first then go from there.
Anthony