The message preview on Galaxy S7 can be shown on the lock screen, status bar or as a pop-up window. But to turn off and disable message preview on Galaxy S7, you just need to do as follows:
1. Open the Messages app on your S7 or S7 Edge
2. Go to the Messages Settings, by tapping the 3-dot icon followed by the Settings. (You can also follow this path: Settings > Applications >> Application Settings >> Messages)
3. Now, tap to open the Notifications settings
4. To manage and disable message preview on Galaxy S7, toggle off both pop-up display and preview message buttons.