Hi!
One idea would be to have an screen with a textbox and have the tester write a combination of nunmbers and letters there.
One way to prevent every user from using that code would be to store different cheat combinations in a database and once a tester enters one of them mark it as used, so one else can use it.
This would require a little work from use since using one combination renders it unusable so you would constatly have to provide new ones.
Maybe, i don't know if this can be done: the first time a cheat code is used you could store in the database, the device-id that used it. The next time you could just check if the device-id corresponds to the one stored and if the code entered is the same.