Connection.update() should do the trick for you. The only gotcha I can see is that update() returns an UpdateQuery object, on which you need to call execute():
try (Connection c = ConnectionConfiguration.to("myDb")
.server("http://localhost:5820")
.credentials("admin", "admin")
.connect()) {
c.begin();
c.update("delete data {graph <http://gra.ph> {:a :b :c}}").execute();
c.commit();
}