Per Billy - removing the plugin from your scanned vst folder -> reset/scan should have worked.
I have another approach I took once to get rid of a pesky plugin ghost. I renamed the standard VST folder to some else like VST2 (something Sonar would not scan). Created a new VST folder with nothing in it. Opened X1 and reset/scan the new empty folder. Of course at that point I had no plugins available in X1.
Once that was done I put everything back to its' original form. (deleted the new "empty" vst folder / renamed the original VST folder back to its' original name. Opened X1 - reset/scan and all was well.
It was my version of grabbing the paddles at an ER and yelling "clear"
:)
There is a couple of other ways I could have done this. My thought was to move the VST folder and its' contents to a safe place. And have Sonar scan a new plugin free VST folder to reset itself. I was chasing a ghost like you have been. And that clumsy approach worked.
YMMV