HELP! Blog author's display name changed by vindictive reader!

Last night I wrote a mildly controversial blog post about religion on my weblog, and this morning I woke up to find that my display name had been changed to the name of a fanatic religious leader. The name was actually added to the user_meta database; deleting that value reverted my display name back to my username.

How could this happen and what can I do to protect against it?