For XP clients, setting (Disabled by default) the policy to "DISABLED" for
Computer Configuration\Administrative Templates\Windows Components\Terminal
Services\Do Not Allow New Client Connections should do it. Reboot or
refresh policy after making the change. The change owuld need to be made
from an Adminpacked XP machine or 2003 server.
Alternatively, you can do this using WMI if the clients support it (W2K, XP,
W2K03 do). You would be using the WMI console's RDTOGGLE command from a
command prompt.
WMIC RDTOGGLE - Turning Remote Desktop listener on or off
remotely.