I'm not a techie and wouldn't own a cell phone but, if I did, I'd rip the sound card out of it. That oughta make it stop. Might wreck the phone, too, but meh.
I repair stuff all the time. Go in and clip the little tiny wires to the button speaker marked 'speaker'. However be advised, this turns your phone into an expensive 'camera and texting pager' only.