One of the most reliable methods of removing ink from leather is to gently rub the area with rubbing alcohol in a circular motion. Once the stain has completely disappeared, use leather conditioner to bring back the original shine and moisture of the area.
Should this not work, spray the stain with hairspray, and allow to sit for a few seconds. Do not let the hairspray completely harden. Rub the area with a damp cloth.
The longer that the ink remains on the leather, the harder it is to remove the stain. Treat the stained area as quickly as possible so that the ink does not permanently set in to the leather. Set-in stains require more harsh methods of removal that could potentially damage the leather or produce toxic fumes.