Mac App Store applications (like Ring Menu) are not allowed to close other applications. That's why you need to Download and Install Ring Menu Toolkit from this website. After you have installed the toolkit the option to close other apps will appear in the right click menu.
First of all, make sure you have the most recent version of Ring Menu and Ring Menu Toolkit installed. Then, right click the Ring Menu background and select New Apple Script. Afterwards, right click on the new item and click Edit to copy and paste your own script. You can also change the icon by dragging a new image onto the old icon in the editor.