The Welcome Back message you are seeing is an Apple notification indicating that the game has connected to Apple's Game Center. If you would prefer to not use Game Center for its leaderboards or to save your achievements, please follow the instructions below to turn off the feature.
Go to your device's home screen and tap the "Settings" app
Select "Game Center" from the menu
Tap your "Apple ID" and then select "Sign Out"
If the message still appears, you may need to restart your device in order to clear it from the device's memory.