You mean delete the account on the voicemail system (which system by the way.. unity, unity connection, unity express or a 3rd party system), remove the voicemail access from the user or delete a user's messages in the voicemail store?
Unity has no APIs.. but since Unity is integrated into Exchange, you can access exchange to perform the deletion. It will require some extensive reading up on the Exchange SDK. I suggest this as a starting point: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/e2k3/e2k3/_exch2k_searching_folders_http.asp (as it happens, the client who ordered the personal directory lookup suddenly had a change of heart now and now I have to access Exchange.. so that's how I got that link). Prepare to enter the world of WebDAV. There are other means, but WebDAV is the most modern and multi-purpose (as in you can use it from non MS environments using non MS programming languages).
As for third party systems.. you have to read their documentation and figure out how they work.. keep in mind that in the Cisco world, VoiceMail and CallManager are very loosely connected.. it's not like you can tell your callmanager to list available voicemails and the likes.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.