Squirrelmail is just a front-end to imap, so I think you need to disable imap access to root. What does your
authmodulelist look like? If it's
Code:
authmodulelist="authmysql"
what's wrong with deleting root's record from the mysql table? If you need a record for root for some reason, you could disable imap for root with something like "disableimap=1" in its "options" field (you'll have to look up the exact syntax), or by giving root an impossible "crypt" field (like the empty string).