Did you use the built in wizards to add users to your network? If so, then, by default, they will be administrators to their local computer. This is not the same as a domain administrator. If you want them NOT to be an administrator to their local computer, and you understand that there is software that may not run without you making additional changes to their workstations, and that they will not be able to install Microsoft Updates etc, then you can logon to each workstation with administrator rights and remove them from the local administrators security group. To do this, I would right click on My Computer Icon, select manage, then Local Users and Groups, Groups, double click on administrators and when you see the memors, just high light the individual and select remove.
Let us know how it goes.
best of luck