For being such a durable material, leather can be very temperamental when it comes to cleaning it. Leather needs to be handled with great caution as it is adversely affected by many substances. If ink happens to find its way by way of stray pen or any other means onto a leather purse there is no need to worry. Leather may be tricky to clean, but it isn't impossible.
Spray hairspray onto a soft clean cloth.
Wipe the ink stain gently with the cloth, applying more hairspray as needed.
Wipe the area in a gentle circular motion.
Allow the purse to air dry in a cool dry place.
Repeat if necessary.
Test the hairspray on an inconspicuous area of leather first.
Do not use milk or toothpaste on the leather. Avoid the use of abrasives and harsh chemicals.