Enough Is Enough

"Enough Is Enough" was the title to, or significant lyrics from, a song by Donna Summer and Barbra Streisand. The Doctor once quoted the words when trying to stop a round of bickering between Col and Professor Ty Benson on the planet Sunday. (NSA: Wetworld)
 * In the real world, the song was originally entitled "Enough Is Enough", but then changed to "No More Tears". Wetworld offers no firm statement that "Enough Is Enough" is the title, as the Doctor is merely quoting the lyric, and crediting the artists who made it popular.  It is likely no coincidence that the reference is made in a book named Wetworld, since the song appeared on Streisand's album, Wet'', whose songs are united by the theme of water.