elias_t's functions sound like the way to go but if you want a quick, dirty, hacky way to remove a tri - and it's NOT sharing verts with any other tri - simply set the coords of one of the tri's verts to match the coords of another of the tri's verts. This will cause the tri to have zero surface area and so can't (and wont) get rendered.
Uh, I feel so dirty! :)
Uh, I feel so dirty! :)