Can a university retract a degree for actions taken after the degree is awarded?

It would depend on the jurisdiction. In the US, you have a property right to the degree that you earn and pay for (regardless of the ultimate source of funding -- the student has the responsibility to pay), and as long as you don't violate the conditions for obtaining the degree (various forms of dishonesty in admissions and satisfaction of the degree), improper actions after the fact don't license depriving a person of what they have earned. In Europe, the legal basis may be different. Whereas in much of Europe state law governs revocation, in the US revocation is governed by university rules.

In the case of Dutch psychologist Diederik Stapel, the thesis data had been destroyed so it was impossible to prove fraud in the thesis, and also impossible to disprove it (Stapel short-circuited the university's proceedings by returning his degree). It appears that there are, in The Netherlands and possibly other European nations, vows that you must take in order to receive the degree, to act honorably (defined locally). "Not committing fraud in the future" is a likely candidate for such a vow.

Yes. Although you will need to do something blatantly unethical.

For instance, my alma mater (the University of Konstanz, in Germany) revoked an alumnus' Ph.D. after this alumnus blatantly falsified data, although the Ph.D. thesis as such was not tainted.

In the vast majority of universities from the Spanish speaking world the degree is not revoked even when they learn, after the degree is granted, that the holder plagiarized in the thesis that was presented for that degree. This is sad because the lack of academic integrity by the universities that have been put on the spotlight affect the region as a whole.

For example in Guatemala, the University of San Carlos learned in 2014 from political magazine Contrapoder that then presidential candidate Manuel Baldizón plagiarized his PhD thesis. As a result the university assembled a council to decide on how to proceed. After deliberating for over 60 days, and without even looking at the evidence!, they decided that due to the lack of internal regulations there was nothing that they could do, therefore Baldizon’s PhD was upheld and valid.

Back in 2013 the University of Valencia, Spain came to the same conclusion. Except that they did not even form a council.

There are many other cases like this through Latin America. The latest high profile case is in Peru. Former presidential candidate César Acuña has been accused by the press for plagiarizing the thesis that granted him a PhD from the Complutense University of Madrid, Spain.

In the Dominican Republic current president Danilo Medina not only plagiarized the thesis the earned him his bachelor’s degree, he did not even complete the credits for the undergrad program. This case came to light when he was still a candidate. He reached office. And the person that denounced him was fired from his administrative post at the Autonomous University of Santo Domingo.

In Mexico, Boris Berenzon from the National Autonomous University was fired due to “inadequate academic practices”. Not only did he plagiarize both his Master’s and Doctorate’s thesis but also books and dissertations that he presented as his own in international forums. He was fired but his degrees, from that same university, have not been revoked.

One exception is from The Colegio the Mexico. They revoked the PhD degree from Rodrigo Núñez Arancibia after it was proven that the thesis he wrote to earn him that degree contained plagiarized material. And he was a fired from his post at the Michoacan University of San Nicolas Hidalgo.

While it took less than six months to find Núñez guilty of all chargers, it took more than 10 years to find that Mr. Berenzon’s academic practices were substandard. What is different in Núñez’s case from Berenzon’s? Why was the degree revoked from Núñez and not from Berenzon? Their respective thesis were just one of many of their works that contained plagiarized material, they both stole the work of other authors for books and arbitrated articles that they signed as their own. What is different then?

Mexico is very sensitive to criticism from abroad, especially from the USA. While those protesting Berenzon’s frauds included students and domestic authors, Núñez plagiarized material from North American authors. Professors Susan Schroeder from Tulane University; Stafford Poole from St. John's Seminary College; Sonya Lipsett-Rivera from Carleton University; John Chuchiack from Missouri State University, and Martha Few from the University of Arizona, reported that in 2014 Nunez plagiarized in its entirely the book “Religion in New Spain”, a book from University of New Mexico Press. This prompted an immediate scrutiny of all of Núñez’s work, including his PhD Thesis.

It is unfortunate but turning a blind eye to thesis plagiarism by some universities in Latin America and Spain questions the integrity of all degrees coming from that region. If you read Spanish check www.plagios.info