Currently, you cannot dynamically enable/disable the editor features depending on the user role. You can remove HTML mode by modifying the ConfigFile.xml (located by default at \Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\18.104.22.168__1f131a624888eeed\RadControls\Editor\). However, this change will affect all editor users.
If you have experience with Visual Studio and C#, we can assist you in creating your own editor (which inherits from the RadEditor for MOSS). This way you will be able to implement role based or even page based security and customize the editor for your site.
the Telerik team