decide against

痴漢をはたらいたとされるNHKの森本アナウンサーが不起訴になりました。これにより今度はNHK内部での判断が下ることになるでしょう。
No charges for NHK anchor assault

The Tokyo District Public Prosecutor's Office has decided against filing an indictment against Takeshige Morimoto, a well-known NHK announcer arrested last month for assaulting a woman on a Tokyo train, investigative sources said.

The victim has not indicated a desire that he be punished, the sources said Monday.
Morimoto, who anchored a nationally televised morning news program on weekends, was arrested on the night of Nov. 14 after allegedly slipping his hand into the underwear of the woman and touching her breasts. He told investigators at the time he was drunk.

http://www.japantimes.co.jp/text/nn20121212a8.html

decide against〜は検察が不起訴に決めた場合によく見かける表現です。「〜することに反対する（against~ing）ことを決める（decide）」、つまり「〜しないことに決める」という意味になります（例：I decided against attending his class.）。もってまわった感じもしますが、これが法律の言葉なのでしょうね。（UG）