I'm in the pre-planning phase of coming up with a game design and I was wondering if there was a sensible way to stop people from editing saves in a game with offline and online components.

The offline component would allow the player to play through the game and the online component would allow them to play against other players, so I would need to make sure that people hadn't edited the source code/save files while offline to gain an advantage while online.

Game likely to be developed in either .Net or Java, both of which are unfortunately easy to decompile.