Plans for a movie remake were announced in November, before writer Whit Anderson quit the movie, leaving the project on hiatus.
Head played the role of Giles on the TV series opposite Sarah Michelle Gellar, and said that he would not even watch the reboot if Whedon was not involved.
On whether he would reprise the role of Giles, he told PA: "Not if Joss Whedon wasn't involved no. No, absolutely no!
"It's Joss Whedon's project, it's his baby, why anyone would think they could make a film without him I don't understand. It's just illogical."
The British actor had previously described the movie reboot plan as a "hideous" idea.
Anthony Head currently stars as Geoffrey Howe in The Iron Lady opposite Meryl Streep.
Watch Anthony Head in the Iron Lady trailer: