Seem some posts around this recently, and this was kind of hard for me to find so I thought I would share. 

"%programfiles%\UltraVNC\unins000.exe" /VERYSILENT /NORESTART

If you try to do just /SILENT, there is a prompt to ask you to reboot, so it's really not silent. 

In a script, i did it like this:

Directory: C:\Program Files\UltraVNC
File: unins000.exe
parameters: /VERYSILENT /NORESTART