HKCU\Software\ORL\VNCviewer\MRU

Long answer

In UltraVNC it keeps a list of the host you connected to before in its little drop down.

Over time it tends get cluttered with one time use entries making it more difficult to pick the right one.  After it became enough of a burden I opened up Process Monitor to see what I could see.

After dialing in the filtering so I was only looking at vncviewer.exe I can see its writing to the registry in a directory called setting – that sounds promising.  Opening that up and routing around a bit reveals this.

Which looks suspiciously like the dropdown.  Basically you have some name/value pairs for the host names.   And index specifces their order.  And you can just delete any entries you want provided you also remove it from index.

And now I can sit back and enjoy my clutter free pull down.