The 'SketchUcation Toolset' also has an 'Uninstaller' in its submenu.
If you use that tool it renames the 'loader' .RB in your Plugins folder as .RBX so it then no longer loads or even gets listed in the disabled plugins in the 'SketchUcation Plugins Manager'.
Provided that the RBX still exists it appears in the 'Unistaller' dialog and can be 'reinstalled' if desired.
However if you open your Plugins folder [also possible by using the SketchUcation Toolset's submenu item] and find the now uninstalled RBX version and delete that file it's no longer listed.
For tidiness you might also want to find any matching subfolder using the same name [common in many newer Extensions], and delete that too...
Of course, to see these changes you'll need to restart SketchUp, because dynamic unloading an extension etc is NOT possible within a current session...