We have a long-time user on Travel.SE who has, for whatever reason, gotten overly frustrated, changed his account name, and unaccepted everything, wiping his answers and turning his questions into garbage. What's the accepted process for dealing with this as a moderator? :(
Revert his edits. If he continues to vandalize his posts, a suspension would be appropriate.
In general, the content license on Stack Exchange prevents users from taking their ball and going home. Their name can be removed from their contributions (either manually or by deleting the account, if that's what they want), but they can't revoke the permission they gave us in the first place to republish the content.
More details on the licensing here: Why can I only delete 5 posts in a day? Is this legal?
Now, this naturally applies mainly to stuff you want to keep in the first place. If someone's deleting their posts and they suck, you don't have to take the hard line. :)
While you're restoring content, also don't forget to start a dialog with the user. It's fairly rare for an established user to just up and rage, so there's usually something going on that triggers their outburst and often can be fixed. See Thorsten's answer for more.
The very first question which comes into my mind:
I cannot know what exactly happened because Mark does not say what happened, but destroying content means that (if the user is supposed to act normal) feels to have been treated badly by SE.
Which can have several reasons and must not be discussed in detail, but if it is a long-time user and he provided valuable answers (you do not want that his answers are deleted, else you would not care anyway), there is something wrong.
I miss something from the answer of Anna because it seems detached from the user itself, it only views it from a moderator perspective.
One problem with this "This is now our content and you do not may change it !" approach is slow-poisoning: If someone knows that SE moderators try to enforce valuable content, users may try to poison their answers slowly over a longer time so the former valuable content transforms into junk.