I did not read user agreement - maybe you did? But!, does the content of the answers belongs to a given user as intellectual property? Such, that when user is suspended he can demand to remove his answers from the site in order to be fair? Interesting topic for a website with private ownership.
To sum up long and interesting discussion below:
SE is licensed under a CC-wiki license with Attribution required. You can read it at http://creativecommons.org/licenses/by-sa/3.0/
There is a post on S[OFU] about same topic with more details: Who owns the copyright to S[OFU] content?
Stack Exchange can use your work (user-generated content) to earn money. This is written in the short summary of the license hyperlinked above.
Effectively you retain only credits on your work. But not the rights to remove it from circulation or issue a new license which will contradict / constrain CC-wiki (this part is declared in termination section of CC-wiki).
License is effectively infinite (up to the limits of international law, about 60 years)
There is no way to terminate this license by the will of the creators (users).
Good news. If we look at the UK CC license (lest assume our user lives in UK), we read the following (http://creativecommons.org/licenses/by/2.0/uk/legalcode):
"2.3. This License does not affect any rights that the User may have under any applicable law, including fair use, fair dealing or any other legally recognized limitation or exception to copyright infringement."
So if I understand correctly - when a user gets banned from this website he can remove all his work from here. At least in the UK. Am I correct?