I believe that there is no option that gives you such a setting.  You may have to handle the client side BeforeSelectChange and cancel the change based on some setting that correlates to the current user.

I want to be allow editing of data to certain users and disallow it to others depending upon their security status without using two different controls, but this is kind of what I thought. Making it not editable still allows them to change the value of the control using the dropdown and disabling makes it so grayed out that people can't read it. I don't know why Infragistics gives their stuff so many frivilous options that you can't find the commands you really need easily, yet they neglect to include things that would be very useful.