All App Inventor apps now have a default About Screen, which is accessible via the Menu button of the device.
What you can do is to add your own text with the new Screen.AboutScreen property. As footer the following text will be displayed as default:
invented with MIT AppInventor appinventor.mit.edu. You do not have the possibility to remove that text.
Thank you Hal for this tip: Just use a html comment <!-- to uncomment the default text!
You also might be interested in Tony's solution to disable the hardware menu completely. Thank you Tony!
This work by Pura Vida Apps is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License
with attribution (name=Pura Vida Apps and link to the source site) required.