The first time a player reads or deciphers the magic word or phrase that activates a magic item, pass it to the player in a note instead of telling them verbally. If the player reads it aloud and the item is being held, the item will be activated instantly regardless of the player's intentions.
"Say, John, what does the note say?"
"Well, Jane, it just says, 'Disintegratrix.' Whoops!"
Naturally, this does not apply if the player character is being told the magic word or phrase by someone (or something) that knows.
"O wise and learned sage, we have paid you the sum you require. Pray, tell us the magic word."
"The magic word is... 'Kaboom.'"
"Did you say, 'Kaboom?'"