A click on "Unlink" does work but you still need to place the link between PLAIN or CODE tags. Example: After marking of the second line in the editor and a click on "Link" we can see this: If you click on "Unlink" it is simply removing the link. Result: But it will still appear as a link in the Preview window. As mentioned above you need to use PLAIN or CODE tags.
I think you can get away with just adding the first [P____] tag. The second [/P____] should get added automatically from memory.
This will work as long as no other formatting is being used in the posting. But it might look like this in the Preview window if [/PLAIN] has been left out:
I would like to add a few things, if you folks don't mind. First: In your Preferences Tab > Options you would have needed the option enabled: "Use the rich text editor to create and edit messages" Without that option enabled, you will not see the "delink" button. Second: Where are those "link" and "delink" buttons? Here: