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:
Start | Run | Gpedit.msc if editing the local policy or chose the appropriate policy and edit it.
Computer Configuration | Windows Settings | Security Settings | Local Policies | User Rights Assignment.
Find and double click "Deny logon through Remote Desktop Services"
Add the user and / or the group that you would like to dny access.
Either run gpupdate /force /target:computer or wait for the next policy refresh for this setting to take effect.