Ensure your object has a copy constructor that allows you to make a deeply-cloned copy of your class.
Then call this when you return the object, e.g.
SomeClass instance = // ....
public SomeClass getInstance() {
return new SomeClass(instance);
}
This won't make the returned object immutable. But it doesn't need to be - you just don't want the external code making changes to your copy of the data.