When you are flowing portraits, you will be shown a preview of the portrait flow.
Here, you can click on the individual portraits to tell Pictavo not to flow anyone in that location. If you decide that you want to put someone in a checked box, just click on the empty box and the portraits will reorder. Thus, if you check boxes like this:
It will produce a portrait flow that looks like this:
If you wish to put someone back in the boxes, you will need to reflow the portraits if they are already on the page.