You can use the EditText.TextChanged event. This is called every time the text in the EditText box is changed so all you need to do is check for a CR in the new string, and if there is one revert back to the old string:
B4X:
MyEditBox.TextChanged (Old As String, New As String)
If New.Contains (CRLF) Then MyEditBox.Text = Old
Return
You will need to check the syntax for using CRLF, as I am not sure this is correct.