To open your user Plugins folder [i.e. the folder where extensions and plugins get installed by default], you can use this snippet in the SketchUp > Window > Ruby Console:
UI.openURL("file:///#{Sketchup.find_support_file('Plugins')}")
You can of course 'navigate to the equivalent folder - although on a PC it's hidden by default:
C:\Users\YOUR_NAME\AppData\Roaming\SketchUp\SketchUp 2017\SketchUp\Plugins
For other SketchUp versions change the version suffix - e.g. 2016...
Then find the .RB 'loader' file - which creates the extension, and then [re]move it.
Also find the subfolder with the exact matching name, and [re]move that.
Note that some very simple [older] Plugins might not have the matching subfolder...
Alternatively...
The way that the SketchUp ExtensionStore³ 'Uninstaller' works is that it adds an X to the loader .RB file-type - i.e .RBX
This stops it loading, and being listed in the Extensions and Plugins Managers, however, it can be 'Uninstalled' using the toolset - whereby its loader is reverted to .RBB ...