I can't close them, I can't click on the name of the contact, I can't click on the config button and the pop up seems broken.
The sidebar expands to also show their names if you move your mouse over the sidebar.
I had to refresh Facebook several times, clear cache, and try different browsers.
So here is how to hide the Messenger sidebar column from Facebook.
In the show list, click Add-ons that have been used by Internet Explorer to display all add-ons.
But hack can also mean a shortcut, a workaround.