Can a university retract a degree for actions taken after the degree is awarded? I was looking at this question on The Workplace, where in the asker was considering doing homework for students for a fee. My initial thought was that it's possible he went to the same university*, and if discovered they would attempt to revoke his degree. I considered that this would be analogous to failing and expelling a current student who was found to be breaking the academic honesty policy. I know that some universities have recently retracted honorary degrees awarded, but of course, the course work was never done to deserve these degrees. So my question is...

Is it possible for a university to retract a degree that had previously been awarded for a full course completion if the individual does something that would clearly harm the university/violate the ethics of the university?

*this is completely speculative, and the question is theoretical at best
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.