Hello! Bored had some really good ideas, and I would definitely recommend four thieves vinegar! But another way is to try and bind the person, of course following the Wiccan Rede or the moral code you follow, if any. A simple binding spell should do the trick, to prevent the person from telling the secret. If that's your concern. You can make a poppet, and bind it's/their lips with string and say something to the affect of: "With this cord, I implore, and tie you lips forevermore, from telling all ill and bane, and from speaking maliciously of me again." You can maybe add something about the secret which you told, and perhaps change it. I just made that up on the spot, so it needs some work. And for binding, you may want to use the seal of Saturn, if your into using planets, and maybe make a sigil of their name and seal it.
Hope this helped!