Comment by itake

At the start, getting users is the first problem. No one is going to bother scamming your app if there is no one there.

Then once people exploit the app, that doesn't mean they wont add value (e.g. contribute positive content). Maybe they are just a high school kid that wants to talk to his friends in his last town?

Once you have users, then there will be other easy signals to detect: Is the person teleporting? Do they hit rate limits freq? Is their GPS location the exact 'center' of the city? Is there GPS a nice pretty number? Does their GPS location never move?