There is this girl who is singing and I am going to tell her to stop.
Why do you want her to stop singing?
Her voice is not to my standard and she sings too high
I think that you should ask her nicely. It is going to make her feel bad if she thinks that you hate her singing.
She should not have a bad voice in the first place.
I don't think it's very nice to tell her you don't like her voice and I worry about her feelings. I think you should think twice and just deal with her singing, as it's something that probably brings her joy and you shouldn't ruin that.