totally confused please help

today someone has managed to not only change their username
from there joined name to guest but they also dont have any info like when they joined etc infact they dont even show up on the username list

how is this possible when in the config ive put it that users cant delete themselves or change their names?

please advice

are you sure it isnt an actual guest?
do you know what ther user name was before they changed it?
If they are posting, have youmade it so guests can post?
These are the only things I can think of straight off

Yes I know who the person was before they changed to guest it shows on here the person orginal name as she had edited her post
http://allstoxforum.niceboard.com/stockcars-f1/cowdenbeath-19th-july-08-t1534.htm

on the config its always been set as users cannot change their own name or delete themselves and you have to be a registered member to post this is why im so confused ive checked that none of the moderators have deleted her and they say they havent