How to ignore faces permanently in iPhoto?
When you click on "search faces", you will be presented with groups of four faces. Hover with the mouse over the picture of a face, and a small circle with an "x" will appear in the top left corner. When you select this x, this face will never pop up again.

But you have to do this for every instance of that face. If you have hundreds of images with one particular person which you will not add to your known faces, you will need to click on every instance of it.