Make the constructor private.
Uhm, how would you make a instance of this class then?
Why don't you want inheritance? I don't understand!
Answer:
You can create the class without the public declaration. In such a case the access for that class would be default and hence you cannot extend that specific class outside its package. Still you can extend this class within its package.
The purpose of having the final keyword is to ensure that the class is not inherited. doing it without final is like touching your nose around your head when you can do it straightaway...