How can I use the camera in Windows Phone 10 and save screenshots o pictures with out flash (I mean blocking the flash option to the users) Is it possible?
You can create the user experience you want making your own camera app. Universal Windows Platform APIs will give you full control over what your user will be able to do.
Take this sample camera app as a good starting point
https://github.com/Microsoft/Windows-universal-samples/tree/master/Samples/CameraStarterKit
The process of taking a picture using the camera and taking a screenshot are completely different.