I was attempting to deny all access to guests, as we want only authenticated students. I changed the guest role to the third column. (Prohibit ? Prevent ? Well, I can't look now !) I think I'm locked out because everyone gets this role in addition to other roles.
To add insult to injury, this did not keep guests out, it just made everyone a guest, without the option of editing.
This is also seen here
http://moodle.org/mod/forum/discuss.php?d=81170
Roles and permissions
Changed GUEST role, now I'm locked out
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