If you suspect someone of cheating in Fun Run 2 there's an option to report them in the post-lobby after a race. Select the icon to bring up the options:
Then tap the icon next to the player you want to report. This will bring up the Report Player screen:
Tap "Send Report" to report the player for cheating. If a player gets many reports we'll investigate the account. To prevent people from abusing the system you can only send 3 reports in one week.