The prophet Zechariah is recounting that God showed Joshua son of Jehozadak, the High Priest, before an angel and before Satan, who stood at his right hand to accuse him. God "cries out" to Satan not to accuse Joshua (because his sons married non-Jewish women, see Ezra 10:18), since Joshua was a "coal spared from the fire" (explained in Sanhedrin 93a).
The point is not that Satan, the accuser, the angel of death, the evil inclination (ietzer hara) (see Bava Batra 16a) has free will to accuse or not, but that, according to his job, he was supposed to accuse Joshua, which God prevented.