The "Welcome back" message is authenticating your username in Game Center. Game Center is an Apple application, which allows you to post achievements, posting to the Leaderboards and playing Multiplayer games where applicable. The "Welcome Back" message lets you know that Word Warp is using Game Center.
If you do not want to use the features of Game Center in Word Warp, under "Settings" there is a "Game Center" option that can be turned "OFF." I see you have it turned OFF. You will need to turn this option back "ON", if you decide to use Game Center later. Also, go to the Game Center Application icon in the device settings, scroll down the settings screen to Game Center, tap the icon, tap account, tap sign out.
Sometimes after turning it OFF, it is retained in memory on the device until you restart your device, even though you turned the option off.
To restart, press on the power button for 3-5 seconds until you see the bar to power it off. Slide the bar and power the device down. Power it back on, tap back into the application and you should not see the Game Center prompt again.
Refer to Apple Game Center support (http://support.apple.com/kb/HT4314) for information on using Game Center.