Ours was pretty similar to what Tom said. We had something called Special Status, which was for sisters who were student teaching or on internship and really not on campus. They paid a fraction of dues I believe, and could come to formals, mixers etc if they were able.
If you did not make grades or didn't pay dues you could not go to mixers, hold office or vote.
Then we had inactive status, where you were not paying dues but were still a sister. You weren't allowed to come to sorority events unless it was by special invitation (i.e. if a sister asked you to be her "date" for the formal). Going inactive is only supposed to be for things like financial hardship and family problems, although there were a couple instances where people with only 1-2 semesters to go went inactive rather than disaffiliate. Often because our national would not terminate a senior.
We really don't have rules about becoming alum...once you leave school, unless you have disaffiliated you are an alumna. We have a sister who pledged and then left school the next semester. Wherever she is
she's still considered an alumna. As far as paying off debt, if someone has really been a butt-head where $$$ is concerned, they generally know not to come around.