Full disclosure, I tried it before from an external caller using a SIP trunk to IPO server edition in a lab environment and it appeared to work fine. I tried it again from an internal caller and it did not work.
With an addition of a second Generic action, I was able to block it for both external and internal callers. Generic action 2 manipulates the Caller ID Name. Internal callers will match the mailbox by name, so blocking the Caller ID Name prevents VMPro from locating the mailbox of the caller. The only way I found to block the name was to set the $CLI_NAME variable to the $CLI length.
Use at your own risk, I have not tested this in a production environment.
->Start Point to Generic1 action
->Generic1 action to Generic2 action
->Generic2 action to Leave Mail
Edit Generic1 (same as before)
Specific tab:
Select Generic command = String Manipulation
Select Variable = $CLI
Select Variable For Result = $CLI
Select Operation = From Position
Number of Characters = 0
From Position Index = 1
Edit Generic2
Specific tab:
Select Generic command = String Manipulation
Select Variable = $CLI
Select Variable For Result = $CLI_NAME
Select Operation = Length;All