this is not possible. The Unity SA is specifically designed such that you cannot change rights or remove AD/NT/Exchange accounts directly in our interface. This was done to prevent security issues since administrations with COS access to the SA may or may not also have NT/AD rights on objects in the directory. To prevent any possible disasters that ability was specifically removed from Unity's SA very early on (circa Unity 2.2).
I know for voice mail only installations this is cumbersome but for UM installations it's critical that Unity not even be on the short list of suspects when objects go missing or are modified in bad ways in the directory. Without such safety measures many companies wouldn't even consider deploying Unity in their existing directory (which we discovered right around the 2.1 release - hence the modification to the SA to not allow this at all).
