I don't think anybody mentioned this yet, but if it's pure white paper, I have great success using a Q-tip dipped in bleach (squeeze out the excess so it is not "drippy") and carefully dabbing on the ink. Within a couple seconds, the ink disappears like magic. If a Q-tip is too large, I've also used the end of a toothpick. You can use the toothpick "as is" or wrap a tiny bit of Kleenex or toilet tissue around the end and dip in bleach and touch the unwanted ink. The trick is to use a very small amount of bleach.
For colored paper, I've bleached out the ink boo-boo, and then actually bleached a larger area than necessary so it was a white space, then stamped an image in the white space. It looked like it was supposed to be that way. Wish I had photos but I don't.