"How to stop users from manipulating Game Save Data?"
- there is no way to prevent it, only ways to make it "inconvenient"
- if your game is popular, cheats and trainer programs will surely be made, thus it only ends up "inconvenient" for 1 guy who made the trainer
Think of it this way ... hacks are common for Counter Strike, Battlefield, and World of Warcraft. If the $ billions of dollars and collective programming power of Valve, EA, and Blizzard cannot stop cheats, then neither can you.