I handled the Kana situation. He wasn't allowed to threaten the secret with the excuse of "love." He was taught his place, and he has a constant reminder so he won't make the same mistake again.
Now Tohru Honda.
I see no threat in her. Not now. She knows the secret, so she won't love one of them. She won't break their confidence by telling others because she is naïve.
She is safe for now.
But if she causes problems then this is only the beginning, and rest assured, I will finish this. I will finish her if need be.