I agree that their comments should be deleted automatically, however I did not like the message their comments were replaced with because it implies I banned them. It would also be nice if moderators and admin of other instances could at least be notified. If the user has written a really useful answer to a question or a how to guide then that could disappear forever. Someone else in this thread stated that the comments should be deleted forever. I really don't like the idea of someone having the ability to delete things completely off of my server.
This one of tbe reasons I created !dullsters@dullsters.net I do not have to worry about one instances modderation policy or splitting of users. Sure I could go crazy one day, but I have a robust ruleset in place that the comminity has followed without issue.