You absolutely cannot do it by text. In person is best, but at least do it by phone. It is simply not fair to do it in a way where the other person cannot say anything to you. She deserves to hear the reason why, if she wants. There are some other reasonable questions that one may have as well, she deserves to be able to ask them. You're certainly not required to give her answers, but you have to say no to her face, not hide behind a block feature.