How do i get my kitten to stay off the ledge?

Well the way my house is set up there is a ledge where you can see the stairs going downstairs. sophie my kitten, likes to jump up there. But i'm scared she might see or hear something to scare her and fall off because the ledge is very narrow. Or try and get something she sees, jump off and hurt herself. I would spray her but again worried she'll get scared and fall off. i need to train her early while shes only four months. any one have any advice