Method 1: Wisext menu

Versions before 0.700.1:

In the Extensions menu you select  "Uninstall the Wisext tools": it will delete all the plugin files on the disk.

Version 0.700.1 and after:

In the tool sub-menu in the Extensions menu, open the "About the Wisext tools" window and click on "Uninstall all the Wisext tools"

Method 2: Extension Manager

In recent SU versions, open the SU Extension Manager  and uninstall the Wisext extension.

 or 

Method 3: deleting files manually

You can delete the files into the Sketchup plugin folder:

file to delete: wisext_alltools.rb

folder to delete: wisext_alltools

Where is the plugin folder?

Microsoft Windows

For SketchUp 2014 and later:

C:\Users\<your_windows_user_name>\AppData\Roaming\SketchUp\SketchUp [n]\SketchUp\Plugins

For SketchUp 2013 or earlier:

C:\Program Files (x86)\SketchUp\SketchUp [n]\Plugins

Mac OS X

On OS X, the plugins are installed in the plugins directory within the user's /Library/Application Support/SketchUp directory:

/Users/<your_mac_user_name>/Library/Application Support/SketchUp [n]/SketchUp/Plugins