User expire to guest...

[Davey]

I searched for a good 15 minutes in the Beta hacks, in here and in the Full releases, and I couldn't find anything matching the word 'guest'.
I'm requesting this:
When a user is deleted, their name changes to 'Guest'.
This is slightly irritating.
What could be changed to keep the old name in place of 'Guest' but make it unveiwable.
For example:
The username 'Bob' registers, decides he wants to be a jerk and gets banned, and his username gets dropped. I don't want his username in his posts to change to 'Guest', instead I want his username to remain 'Bob', but just not be registered.
Anyone catch my drift?

Dave.