You can remove the button, but it will be for everyone, not just the Terminal Service users. However there is a way to prevent a user from shutting down the server.
Click start >> Run
Type mmc, Click OK
From the Console menu, select 'add/remove snap-in'
Click on ADD
find 'Group Policies' Click ADD, click FINISH, Click CLOSE, Click OK
On the Left, open 'Local Computer Policy'
Open 'Computer configuration'
Open 'Windows Settings' - 'Security Settings' - 'Local Policies' - 'User RightsAssignments"
On the Right Side find..."Shut Down The System"
Remove the check mark from any user you do not want to be able to shut down the system.
Caution, if you are using Terminal Services in 'Administrative Mode' Then every user who logs in via TS will be an administrator. In this case, you will need to remove the 'Shut Down The System' privileges from the 'Administrators' group. Next you will need to add the individual persons you want to give the permission for a reboot or shutdown.
Hope this helps.