Dear Christian, 

in order to show an unchangeable String in the Notebook you can:
1) Create a class attribute and define the String, which should be unchangeable, as the Standard value of this created class attribute

2) Create an attribute and define the String, which should be unchangeable, as the Standard value of this created attribute + add “write-protected” in the AttrRep attribute of the class:f.e.: ATTR "NameOfCreatedAttribute" write-protected