How to disable RDP access for Administrator

This seems to be what you are looking for: http://support.microsoft.com/kb/2258492

To deny a user or a group logon via RDP, explicitly set the "Deny logon through Remote Desktop Services" privilege. To do this access a group policy editor (either local to the server or from a OU) and set this privilege:

1. Start | Run | Gpedit.msc if editing the local policy or chose the appropriate policy and edit it.

2. Computer Configuration | Windows Settings | Security Settings | Local Policies | User Rights Assignment.

3. Find and double click "Deny logon through Remote Desktop Services"

4. Add the user and / or the group that you would like to dny access.

5. Click ok.

6. Either run the command gpupdate /force /target:computer on the command prompt or wait for the next policy refresh for this setting to take effect.