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.