The biggest problem we have is with students who sign up for mailing lists, then stop attending for whatever reason. This was very unpopular. Now we keep accounts active as long as the student continues to attend each semester. At about two weeks into a semester, we get a list of all attending students, and diff it with our user list, and remove accounts of students who are no longer attending. The new problem with this approach is disk usage.
Some students just keep stuff sitting around forever.
It's not an easy solution. Otherwise, you'll have to employ an army of clerks to maintain this stuff.