How to disable the guest session in Oneiric?

By default it isn't enabled. You can disable any account with:

sudo passwd -dl foo

replace foo with the username you login as 'guest' with

Thanks, that's useful information, but doesn't quite solve my question, so I should be more specific:

Oneiric has a feature whereby it temporarily creates a guest session account for you. So if I go to the session indicator menu that shows my name, there is a "Guest Session" option, and choosing it logs you in with a random username like "guestIz278gf". This guest user has no home folder and only shows up in /etc/passwd until you log him out.

So how do I disable this Guess Session feature?

I think I've found it: there's a package called gdm-guest-session installed which seems to provide the functionality I described.

Not installed here, quite strange. I'd remove it and see how you go....

I upgraded from natty, so maybe it was installed there.

However, removing gdm-guest-session makes no difference whatsoever, even after a reboot. I can still use indicator-session to switch to a guest session. Perhaps it is built into the new indicator-session? There's a command 'guest-account' available in bash but I'm not sure which package it belongs to.

Depends which login manager gdm or lightdm, config files are in /etc/lightdm/* or /etc/gdm/*.
But anyhow there is a known bug (linked) when it's disabled in greeter it shouldn't display in indicator.
Regarding (not only) dev releases it's helpful to review bug reports.