The plug-in manager has an exclude list. When plug-ins are added to the list, they still reside in the VST inventory in the registry but no longer display in SONAR plug-in menus or browser.
To add a plug-in to the exclude list: click on its entry in the Registered Plug-ins column then click the Exclude Plug-ins Button.
To remove a plug-in from the exclude list: click the "Show Excluded" radio button, click the plug-in category, click the plug-in name in the Registered Plug-ins column then click the "Enable Plug-in" button.
I believe multiple selections may be processed at a time using the standard Windows SHIFT+click and CTRL+click functions. IIRC, the exclude list is restored to its default during a VST reset.
Another solution is move the dll and vst3 files out of the scan path then run a scan.