At every Landscape vertex, each layer has a weight specifying how much influence that layer has on the Landscape. Layers have no particular blending order. Instead, each layer's weight is stored separately and the results added. In the case of weight-blended layers, the weights add up to 255. Non weight-blended layers are independent of other layers and can have any weight value.
You can use the Paint tool to increase or decrease the weight of the active layer. To do so, select the layer whose weight you want to adjust, and use one of the Painting tools to apply the layer to the Landscape. For Weight-Blended layers, as you increase the weight of one layer, the weight of the other layers will be uniformly decreased. Fully painting one layer will result in no weight on any other layer.
When you are reducing a weight-blended layer by holding down Ctrl + Shift while painting, it is not clear what layer should be increased to replace it. The current behavior is to uniformly increase the weights of any other layers. Because of this behavior, it is not possible to paint all layers completely away. Instead of painting layers away, it is recommended you choose the layer you want to paint in its place, and paint that additively.