It seems quite like that you're referring to David Eddings' The Belgariad, a five-part series later followed by another five-part sequel series, The Mallorean.
Magic in the Belgariad is called "The Will and the Word", and a sorcerer can direct his will and speak a word to cause a magical effect to happen. However, there are limitations caused by the cosmic source of the power - if you try to make something not be, the power will rebound on you and destroy you, instead.
This is explained by the sorcerer Belgarath in book 2. Queen of Sorcery:
"Why don't you just wave your hand and make him disappear?" Barak suggested, making a sort of gesture with his thick fingers.
Wolf shook his head. "I can't. Not even the Gods can do that." [...] When you try to unmake something, your will simply recoils on you. If you say, `Be not,' then you are the one who vanishes. That's why we're very careful about what we say."
In the end of Book 3, Magician's Gambit, the evil sorcerer Ctuchik panics during a magical battle with Belgarath and utters the forbidden word:
The Grolim's voice was a hoarse shriek. "Be not!" he cried out desperately, directing all his terrible power at the stone in the little boy's hands. [...] Ctuchik, his long hair and beard disheveled, stood gaping in wideeyed and openmouthed horror. "I didn't mean it!" he howled. "I didn't - I-" [...] Then he seemed to expand, then contract, then expand again. Cracks appeared on his face as if he had suddenly solidified into stone and the stone was disintegrating under the awful force welling up within him. [...] And then, with a shattering sound that was beyond noise, the Disciple of Torak exploded into nothingness.