The feature can't be turned off but you can edit the registry permissions to prevent the system from being able to update the value if you want. Or you can disable updating for certain e-mail addresses.
See:
http://support.microsoft.com/?kbid=304148
If you do this, and set the SYSTEM permissions to Deny for the UnreadMail folder, you'll want to first go to each MessageCount field in the folders under UnreadMail, and set the value of MessageCount to 0 for each.