iNet Protector can not be disabled by most regular users, however users with advanced skills might attempt to break the software. There are several steps you can take to prevent even advanced users from disabling iNet Protector.
1) You should set regular user accounts to limited accounts (not administrator account) and allow your users to log in as non-administrators only. (as I understand, you have already done this)
2) Advanced users might disable the service, if your system policy allows them to do so. To achieve top security, you should block the management console for non-admin users. There is a method of disabling Service Management console.
Here you can find more info (you will have to edit the Windows registry):
http://www.pctools.com/guides/registry/detail/1303/