In that case, just setup all of your users on the domain using Active Directory Users and Computers. Do not add any local users to the workstations, and make sure the Guest accound on the domain and local computers is disabled. Also, make sure you have a good local admin password on the local machines too. Now, if all of the workstations are members of the domain, then only users you setup can log onto them.
Caution, this does not work with windows 9x and ME as workstation OSs.
I really like Active Directory, makes user/compute administration easy.