No one has ever been stripped of a Nobel Prize, because this is actually specifically forbidden by the organization which administers the Nobel Prizes. According to the Nobel Foundation, “no appeals may be made against the decision of a prize-awarding body with regard to the award of a prize,” and no prizes can be revoked after the fact, no matter how controversial they may seem. Despite the existence of several petitions pushing for retraction of controversial Nobel Prizes, it is unlikely that the organization will change its rules to make a revocation possible.
The committees which administer the Nobel Prizes are very cautious. They investigate nominees very carefully, often awarding prizes 20 years or more after the Nobel-worthy accomplishment. In some cases, the committee has lingered so long over a prize decision that the honoree has died before the prize can be awarded, which explains why some seemingly Nobel-worthy individuals have failed to receive Nobel Prizes.
Some Nobel Prizes have certainly been controversial, especially after the fact. Antonio Egas Moniz, for example, won the Nobel Prize in Medicine in 1949 for his development of the so-called “icepick lobotomy,” a brutal medical procedure which has been widely condemned. Some controversial leaders like Henry Kissinger and Yasser Arafat have won controversial Peace Prizes, along with people who were later deemed morally suspect, like German author Guenter Grass, who later admitted to serving in the SS during the Second World War.
The Nobel Foundation freely acknowledges that the awarding of some prizes has attracted controversy and concerns from the international community. Accordingly, the organization has agreed to make records pertaining to their decisions public, but these records are not released until 50 years after the fact. The argument is that these prizes are designed to be timeless, rather than being swayed by social issues during a particular era, and that therefore some time may be needed for perspective.
As discussed above in the case of Antonio Egas Moniz, the judgment of the Nobel Prize Committees is not always perfect, and in hindsight, some Nobel Prizes probably should not have been awarded. By contrast, however, there are some prizes which were controversial at the time, like Einstein's hotly contested 1921 Nobel Prize, which later turned out to be entirely merited. Defenders of controversial decisions have also argued that researchers do not always have control over what other people do with their inventions and discoveries, and that someone who has committed questionable acts is still capable of contributing to the advancement of culture and the sciences.
Although no Nobel Prizes have been revoked, there are a few instances in which the prize has been refused. In Nazi Germany, several honorees were forced to refuse to accept the prizes due to an edict from Adolf Hitler; Hitler was irritated that he did not receive a Nobel Prize, and decided that if he couldn't get one, no German should be able to. Several people including Jean-Paul Sartre have also declined Nobel Prizes for personal reasons.