In the below example you can put in what your back key should do.
Or put in nothing to do nothing.
B4X:
Sub Activity_KeyPress (key As Int) As Boolean
' BACK key pressed
If key=KeyCodes.KEYCODE_BACK Then
' I want to capture the key here so I return True
Return True
End If
' Returning False signals the system to handle the key
Return False
End Sub