Get the sources, change them and compile your own Sigil. Thats the only way to get rid of Sigil's own entry. The CleanOPF Plugin will eliminate all others, but it cant do anything against Sigil's own entry.
Of course tools can unpack the epub, manipulate the OPF and stick everything together again, but Sigil will regenarate it when you touch the ebook with Sigil again.
CleanOPF Plugin