The practice of decimation has its roots in Roman history as recorded by Livy. It was a form of punishment for mutinous or cowardly soldiers. Soldiers were divided into groups of ten, and within each group, one soldier would be chosen by lot and killed, either by clubbing or stoning. In other words, decimation is the destruction of one in ten. If you look at the word, you can even see the prefix "deci" that comes from the Latin for ten.
"Total decimation" is an oxymoron. If one tenth is destroyed, nine tenths survive. If you want to show that every member of a group was killed, "total destruction" is a more accurate way to express your meaning.
And when you hear someone else say "total decimation," join me in counting to ten before venting your frustration.