A professional-strength hand cleanser can be a good first option to use. These are generally used by auto mechanics to get rid of tough oil and grease stains on their skin, so a little ink stamp should not pose too big a challenge. Use it on the area a couple of times, making sure you rub it over the ink thoroughly, and then wash it off.
Certain types of ink are not water-soluble, so try applying alcohol or nail-polish remover on these. Simply dip a cotton ball in the solution, and rub it over the ink, until the colour begins to lighten, and eventually disappears. Once it is off, make sure you wash your hand with water and soap, to get rid of the leftover chemicals on your skin. Wiping the area with baby wipes or wet-naps is also an option.
Experiment with a couple of products in order to get rid of the stamp. Try using a stain-eraser sponge – these simply need to be soaked in water, and then applied onto the skin. Hairspray is another product you could experiment with – spray it onto the ink, wait for it to dry, and then wash it off with warm water. Hand-sanitizer can also be used in a similar fashion, to try and dissolve the ink stamp.
If you are wary of trying chemical treatments in order to remove the stamp, you can try rubbing shortening onto the area. Smooth it into the skin, and then wipe it off with a clean cloth. Repeat the procedure, until you notice the stamp fading away. Once it is gone, you can wash your hands a couple of times, to get rid of the greasiness.
Finally, if all else fails, resort to concealer. After all the methods you used in trying to get the stamp to come off, the ink must have lightened up a little. Use a heavy-duty makeup concealer to cover the rest, and you will be free to go about your daily business as usual.