Hover over it, and click on 'Unsubscribe from mod' (the red X button). If you subscribed to a mod from the Steam Workshop, you can uninstall it from within the game: Go to the Mods menu from the main menu, and find the mod you wish to uninstall. After all the controversy that surrounded the paid mods initiative by Valve (with its pilot run on Skyrim's steam Workshop), it appears that the internet's outrage and feedback has finally made the company capitulate, and go back on this idea- there will be no more paid mods for Skyrim.
