You don't stop that. The image you see in the front facing camera is a mirror image of reality. This is done because everyone is familiar with looking at themselves in a mirror. When you take the picture the image is converted to what the camera (and other people) see. Hold some text up to the front camera to see what I mean.
If, for some reason, you want a mirror image then there are photo editing Apps which will flip the image left to right. For example, see Photo Editor.