To delete/update all matching records/triples with unknown object value, you can use the
Jena's graph.find() method that takes a TripleMatch object and returns an ExtendedIterator of all
matching triples. Then you can iterate through them and delete/update as you wish.
Alternatively, you can use a JDBC call to use a DELETE statement directly. Note that you will likely need
to create an additional index(s) to speed up DELETE.